It is unquestionably illegal for employers to discriminate against their employees on the basis of race, age, gender, religion, pregnancy and disability.

But when it comes to discrimination toward employees performing family responsibilities, such as running errands for their aging parents or taking time to care for children of their own, the law can get a little murky.

The federal Equal Employment Opportunity Commission will vote today on whether to issue enforcement guidelines on the topic of discrimination based on "work/family balance." The guidelines, which already have been written, will be released today if approved.

Discrimination based on work and family balance, also known as "family responsibilities discrimination," or FRD, is roughly defined as illegal adverse actions taken against employees because of their family responsibilities. Over the last decade, the number of such cases filed has quadrupled, according to a 2006 report by the Center for WorkLife Law at the University of California Hastings College of the Law.

What makes family responsibilities discrimination tricky, however, is that there's no one law making it illegal.

Rather, lawyers draw from 18 different legal theories and numerous state and federal statutes, such as the Pregnancy Discrimination Act, the Americans with Disabilities Act and the Family and Medical Leave Act, to bring such cases.

"In the absence of applicable legislation, plaintiffs and lawyers have used a tremendous amount of creativity," said Joan C. Williams, director of the Center for WorkLife Law.

Of the cases examined in the Center's 2006 report, the vast majority were brought by women and many specifically involved pregnancy. Pregnancy discrimination charges filed with the EEOC and with state and local agencies increased 45 percent between fiscal years 1992 and 2006.

In a 1999 Pittsburgh case, for example, Kathleen G. Hallberg won a judgment against her employer, Aristech Chemical Corp., when she was passed over for a promotion after giving birth to her second child. After repeatedly being promoted as a childless woman, she said she had been asked by the president of the company, "Do you want to have babies or do you want a career here?"

Other FRD cases involve women fired after coming back from maternity leave, or not given the same accommodations as comparable male workers.

In addition, about 7 percent of the cases studied by the Center involved male plaintiffs. In one local case, Gerald Schafer successfully challenged a Pittsburgh Public Schools policy that only females were allowed to take a one-year leave without pay after the birth of a child.

But not all instances of what might be considered "family responsibilities discrimination" are illegal.

In testimony at an EEOC hearing last month, employment lawyer Zachary Fasman testified that "there can be no FRD claim where the employer refuses to consider for promotion all individuals with young children -- both men and women." Companies also can institute strict attendance policies, even though they might affect parents more than other employees.

Because "family responsibility discrimination" only covers cases that already are illegal under existing laws, some employment lawyers question why the EEOC would need to issue separate guidance on it. The agency already has facts and guidance listed on individual types of discrimination, such as pregnancy, sex and sexual harassment.

"It's kind of a new phrase that the government agencies might be using, but it is really referring to people who can file claims under existing laws," said John Quinn, an employment lawyer with Eckert Seamans Cherin & Mellott's Philadelphia office. "The fear of management attorneys is that the EEOC is trying to establish new regulatory requirements that are not found in any federal statutes."

Mr. Quinn also is concerned that "family responsibility discrimination" might make employees feel that they are entitled to accommodations that aren't specified in any law, or that someone fired or passed over for a promotion for legitimate performance issues might chalk it up to discrimination instead.

Ms. Williams said it is difficult to say exactly what situations are and not covered by various laws because each case is dependent on specific facts. In one 2004 case in Iowa, for example, a pregnant truck driver sued her company after she was denied a request to be assigned light duty work or have assistance with heavy lifting. While the company may have been within its rights to deny that request, a federal court allowed the woman to pursue her claim because the company accommodated light duty requests for three males.

Ms. Williams also noted that plaintiffs in FRD cases have historically been remarkably successful. While about 20 percent of plaintiffs win in the general category of employment discrimination cases, which includes age, race, national origin and religion, more than half of FRD plaintiffs are successful, according to the report by the Center for WorkLife Law.

"Discrimination against mothers is one of the few types of job discrimination that is still very open and explicit," said Ms. Williams, explaining the high success rate for many of the lawsuits. "It's not that uncommon to hear employers and supervisors say, either explicitly or in effect, that this isn't a suitable job for a mother."