Starting today, most people employed in the District of Columbia are entitled to up to four months of unpaid leave to take care of an ailing family member, but employers have until spring to start honoring such requests.

Under a new city law that formally cleared congressional review yesterday, all workers except those employed by the federal government can get the unpaid leave, whether they live in the city or the suburbs.

And the family member can include a newborn child or an ill relative, gay partner or roommate.

Advocates of the legislation are calling it the most comprehensive family-leave provision in the nation because of its broad definition of family.

The 16 weeks of upaid leave, with continued health benefits, are also available to workers who are sick themselves.

"It's a great step forward for the workers in the District of Columbia, said Claudia Withers, an attorney for the Women's Defense Fund, which lobbied for the measure, along with labor unions, women's groups and advocates of the elderly.

Private firms with 50 employees or more and all city agencies would be affected initially. After three years, firms with at least 20 employees will be required to provide the same benefits.

An employer can require that a leave request be supported by a health-care provider.

Business groups had vigorously opposed the measure before the D.C. Council, objecting to the city's mandating such a broad benefit and arguing that it would especially hurt the city's small businesses. After Mayor Marion Barry signed the bill in July, the D.C. Chamber of Commerce lobbied in vain for Congress to intercede.

Business leaders said yesterday that large firms are still struggling to figure out the financial impact of the new law, which would take effect next April, and many small companies aren't even aware of it.

Gregory L. Davis, executive vice president of the Chamber of Commerce, said many large companies have leave policies similar to the one mandated by the law for at least some of their employees, but that most small- and medium-sized firms are now going to have to redraft their benefit plans.

"They are going to be hit hard by this," he said.

Some economists say the law will be costly and could discourage businesses from locating in the District. A study by the federal General Accounting Office of a 10-week family leave law found that it would cost about $5 a year per worker.

That, say advocates, is an inexpensive way to keep families intact. In the long run, they say, that is good for business and good for the economy.

But John Jackeumin, chairman of the legislative committee of the Greater Washington Board of Trade, said business owners have many concerns.

They are worried about what happens when a person about to be fired applies for family leave, he said.

They are also worried about litigation because of the law's definition of a family member, and they are concerned that small businesses will find it difficult to keep jobs open for people who take leave, Jackeumin said.

The Board of Trade plans to brief business owners about the law's impact at a special breakfast seminar on Oct. 11.