Mayor Marion Barry's office said yesterday he will sign legislation that requires District employers to provide up to four months of unpaid leave to almost any worker who wants to care for a "family member," whether it be a newborn child, an ill relative or a gay partner.

Advocates of the legislation, passed last week by the D.C. Council, say its broad definition of family will make it the most comprehensive family-leave provision in the nation.

The mayor is expected to sign the Family and Medical Leave Act of 1990 today, a spokesman said. The law, which must clear the congressional review to which almost all D.C. legislation is subjected, would take effect this fall.

Barry initiated the legislation two years ago in response to lobbying by labor, elderly and women's rights groups. The D.C. Chamber of Commerce vigorously opposed the measure, arguing that it would hurt the city's small businesses.

Private firms with at least 50 employees and all city agencies would be affected in the law's initial phase. Distant relatives, gay couples or even intimate friends sharing the same dwelling are defined as family. The 16 weeks of unpaid leave are available to employees who are themselves sick, as well as to employees who want to care for family members.

After three years, firms with at least 20 employees would be required to provide the same benefits.

People who work in the District but live in the suburbs are covered along with District residents. Federal workers and employees of foreign governments are among those not affected.

While on leave, employees would continue to receive the same health benefits they had on the job.

Some economists say the law could reduce wages and discourage some firms from moving to the District. In addition, they say the law might discourage firms from hiring people most likely to take leaves, such as young women and older people. Other economists say that while the law might cause some businesses problems, it would help the District economy in the long term by helping families remain solvent.

Many provisions of the District law are similar to those contained in the national bill vetoed last month by President Bush, who said it would hurt U.S. economic competitiveness and be an unfair intrusion by government into the affairs of business.

Three states -- Wisconsin, Maine and Connecticut -- have laws comparable to the D.C. legislation. But they provide family leave only to workers who need to care for spouses, children and parents, said Donna Lenhoff, legal director of the Women's Legal Defense Fund, which supported the law.

About 1,300 firms in the District have more than 50 employees and 3,000 firms have more than 20 employees, according to city estimates. About 318,000 workers would be protected by the law this year and 370,000 in three years.

Garland Pinkston Jr., director of intergovernmental relations for the mayor, said the bill ensures that working households "do not have to sacrifice employment to deal with essential family health problems."

Clara G. Schiffer, spokeswoman for the National Capital Chapter of the Older Women's League, said the law would help many middle-aged women who belong to the "sandwich generation." These women, who must care for their children as well as aging parents, have difficulty finding new jobs after they quit one to care for ill family members, she said. "That's why it's important that their jobs are held for them."

Ivy J. Young, director of the Families Project for the National Gay and Lesbian Task Force, praised the legislation for giving homosexual couples some of the same rights as heterosexual couples.

Kathleen D. McKirchy, spokeswoman for the Metropolitan Washington Council of the AFL-CIO, said Western European countries have had paid leave for at least 20 years. "It's been a knockout fight in this country just to to get unpaid leave."

The D.C. Office on Aging reports that 90 percent of the city's nearly 100,000 elderly citizens need family or friends at some point to provide personal or health-care assistance.

But Pedro Alfonso, president of the D.C. Chamber of Commerce, said the bill places an "undue burden" on small businesses and will make it impossible for them to remain competitive and survive. He said the shrinking U.S. work force is making it more difficult to find temporary workers, and that small businesses would be left with no one to perform the duties of absent employees.

He also criticized the bill's definition of family as too broad, entitling employees to use family leave for the care of "the most distant relatives and in-laws, as well as for present or former roommates or lovers."

Paul R. Wall, vice president of Phillips Seafood Restaurant, which owns Phillips Flagship in Southwest, said the law could send some of the city's small, struggling businesses into bankruptcy. "It will be like another tax," he said. "To put another mandated law on their backs will be another reason they can't stay in business."

Two years ago, the U.S. General Accounting Office did a study of the costs of a 10-week family-leave law and found that it would cost employers about $5 a year per worker. It found that many firms actually experienced savings because no wages had to be paid to the absent workers.

Sigurd R. Nilsen, assistant director for employment and training issues for GAO, said firms tend to hire temporary help for about one-third of the workers who would take such a leave of absence. He said the study showed that employers usually replace low-level workers only and that temporary workers often cost employers less than permanent employees. HIGHLIGHTS OF LEAVE ACT

FAMILY LEAVE An employee is entitled to 16 work weeks of family leave during any 24-month period for:

The birth of a child.

The placement of a child with the employee for adoption, foster care or for whom the employee permanently assumes and discharges parental responsibility.

The care of a family member of the employee who has a serious health condition.

The family leave may be taken intermittently when medically necessary subject to certain restrictions.

Vacation, personal or compensatory time the employee elects to use for family leave counts against the 16 weeks. MEDICAL LEAVE

An employee is entitled to medical leave for a maximum of 16 work weeks during a 24-month period.

The medical leave may be taken intermittently when medically necessary.

Any medical or sick leave the employee takes counts against the 16 work weeks. CONDITIONS

An employee must have been employed by the same employer for one year.

Family member means a person to whom the employee is related by blood, legal custody or marriage; a child placed with an employee for whom the employee permanently assumes and discharges parental responsibility; or a person with whom the employee shares or has shared within the last year a common residence and with whom the employee maintains an intimate relationship.

The employer may require that a claim for family or medical leave be supported by a health-care provider. The employer also can require a second opinion, at the employer's expense.

Employees do not lose benefits or seniority they had at the time the leave began.