LEGISLATION TO require that health insurance contracts provide greater "parity" in coverage of mental and physical illness if they cover mental illness at all may well pass in this Congress. A limited and mostly hortatory version of the bill passed a couple of Congresses ago. The idea has bipartisan support; Pete Domenici and Paul Wellstone are the leading sponsors in the Senate. The president has climbed aboard the bandwagon, if wagon it is. The opposition, while real, is somewhat muted, perhaps reflecting the fact that this is a hard cause to which to say no.

So it could happen, and the benefit is clear enough. Mental illness can be every bit as debilitating as physical illness; many people, including, it would seem, many members of Congress, are familiar with cases in which it has struck. It is susceptible to treatment and can often be managed if not "cured." But insurance policies, no doubt mirroring a view in the society at large, traditionally have been written in such a way as to make them far less hospitable to mental health claims than to claims for treatment of often less-serious physical illnesses. The Domenici-Wellstone legislation, and the executive order the president issued the other day, affecting coverage under the federal employment health benefit program, seek in various ways to reverse that.

We are not disposed to argue, so much as to note that in this, as in all such cases, the benefit is accompanied by costs. Both sides in this case seek to have the cost issue both ways, and neither can. Proponents say the need is great -- great enough to require regulation, surely -- but that the cost would be all but imperceptible -- an extra percentage point or two on premiums, perhaps. The business groups that have led the opposition say the opposite -- that the cost would be appreciable, while the need can safely be left to the market. Our own sense is that the two go in tandem.

Opponents -- some, at least -- say the cost increase would be enough to price some significant number of people out of the insurance market, thereby reducing on balance the coverage the proposal sets out to expand. The patchy data that exist suggest that this is an exaggeration -- that by itself the mental health proposal would not have that great an effect on costs (though as drafted the legislation could induce some companies to free themselves from the constraint by dropping mental health coverage entirely).

But the question is whether this will be the only such benefit that, over time, Congress is tempted to confer through the regulation of private health insurance, a relatively new field for it, or whether there will be more. What about the convalescent and other long-term care that many health insurance policies don't cover? Should they be required in the name of parity to offer more? You can think of all manner of sympathetic examples of your own.

The other day in Congress some members introduced a bill to require that contraceptives be covered by plans that cover prescription drugs and devices. It too is presented as a question of unfair discrimination, and perhaps it is; the sponsors note that women of child-bearing age end up having to spend far more than men in out-of-pocket health care costs. By itself, this worthy benefit wouldn't be that costly, either. But is Congress, acting as a kind of national benefit commission, the right forum for deciding what private health insurance contracts should be made to cover when Congress doesn't have to bear the cost nor bear directly the blame for raising cost? It seems to us that question has to be a part of the debate as well.