Nearly one-third of American adults will suffer from a mental disorder or drug or alcohol abuse problem sometime during their lives, a new study shows.

In a survey of more than 18,500 people age 18 or older in five cities, the National Institute of Mental Health found more than 15 percent had experienced symptoms of a mental or substance abuse disorder during the previous month. Additional data showed one-third of the population will suffer from such a disorder during their lifetimes.The study found the most common forms of mental illness were anxiety disorders, such as phobias and obsessive-compulsive behavior, and affective disorders, which include depression and manic-depression.

About 7.3 percent of those surveyed indicated they had problems with anxiety disorders in the previous month, and 5.1 percent reported signs of affective disorders.

Schizophrenia was less common, with less than 1 percent of respondents suffering from such symptoms in the one-month period.

Alcohol and drug abuse problems were common, with about 6.8 percent of people in younger age groups showing addictive patterns. The study found substance abuse among women dropped sharply when they reached their mid-20s, but remained high among men in all age groups.

The survey found severe mental impairment, such as that caused by Alzheimer's disease, increased markedly with age. About 15 percent of people over 85 had such problems, compared to 2.9 percent of people age 65 to 74.

The five cities included in the survey, which was published recently in the Archives of General Psychiatry, were Los Angeles, Baltimore, St. Louis, New Haven, Conn., and Durham, N.C.