Teenagers understand how AIDS is caught but don't know how to avoid getting other sexually transmitted diseases. Many drink to excess, use drugs, eat badly, neglect to wear seat belts and ride with drinking drivers. A substantial number have seriously considered suicide.

Those are the results of a survey, sponsored by health education groups and the Health and Human Services Department, which showed that teens often make wrong decisions about their health and safety even when they know better.Dr. Robert E. Windom, assistant health secretary and head of the U.S. Public Health Service, called the results "somewhat dismaying."

"The results give us fresh insight concerning what our nation's teenagers know and how they act concerning health and sex-related issues - but they also show that too many young people, even when they know better, don't always make the right health decisions," Windom said.

A margin of error for the survey was not immediately available, said spokeswoman Lisa Clough, but about 11,000 eighth- and 10-graders from a nationally representative sample of more than 200 public and private schools were surveyed.

The survey, conducted in the fall of 1987, showed that alcohol and drug use continue to be problems among the nation's young people.

Twenty-six percent of the eighth-graders and 38 percent of the 10th-graders said they had had five or more drinks on one occasion during the two weeks preceding the survey.

About one in 10 adolescents said they had smoked marijuana during the past month, and one in 15 said they had used cocaine during that period.

Most teens understand that acquired immune deficiency syndrome is transmitted by sexual intercourse and drug needles. They know that condoms are effective in helping avoid AIDS and believe they should be used.

The survey noted that 2.5 million teenagers are infected with sexually transmitted disease each year and that many teens cannot identify common early signs of the infections. A majority of those surveyed do not know that taking birth control pills is ineffective in avoiding such diseases.