DECIDING how mental illness should be treated — and at what cost — is no easy matter. Unlike some physical ailments for which there are reams of studies suggesting a relatively clear standard of care, there is often little accepted medical evidence to support the range of treatments for many mental illnesses, like schizophrenia and severe depression.

“It’s very different from the approach to a bypass procedure or a hip replacement,” said Karen Ignagni, the C.E.O. of America’s Health Insurance Plans, a trade association representing the nation’s health insurers.

At issue is not coverage for run-of-the-mill care like prescription medications for depression or a few visits with a therapist. Insurers generally cover these costs the same way they cover medications for, say, high blood pressure.

But when patients need months of residential care, for example, or meetings with a therapist several times a week, insurers balk. The insurance executives say that the medical benefits of such treatments are not clear and that the industry is essentially being asked to write a blank check.

Mental health accounts for a small part of total health care spending — by one estimate, $113 billion annually, or less than 6 percent of the $2.6 trillion overall health care bill. But pressure is intensifying on insurers under the Affordable Care Act, which includes mental heath care as an essential benefit, because they are already trying to keep the premiums they charge for plans on the new state marketplaces as low as possible. Insurers are concerned about the potential for new costs, while patient advocates worry that mental health will be neglected.

Both sides say Washington is partly to blame. The federal government has yet to write the mental health act’s final regulations for insurance companies, leaving a crucial gap between the intent of the measure and how it actually works.

Senator Kennedy’s son, Patrick J. Kennedy, the former congressman from Rhode Island who was one of the law’s main backers, said he worried that the Obama administration had delayed the rules because officials were preoccupied with the president’s broader legislation and needed the insurance companies’ support.