Suicide is now the third leading cause of death among Americans age 15-24, with a more than 40 percent increase among young people in eight years.

The suicide rate for that age group jumped from 8.8 deaths per 100,000 population in 1970 to 12.4 per 100,000 in 1978 (the latest figures available), according to last week's Morbidity and Mortality report from the national Centers for Disease Control. During those eight years, 39,011 people aged 15-24 committed suicide (89 percent white), with white males as the only category "to show a clear upward trend," said the report. The male-female suicide ratio is 4 to 1. The rate for the remainder of the population remained stable.

The federal health agency also reported that homicide, the second leading cause of death for the 15-24 age group and the primary cause for young black males, had dropped more than 25 percent between 1970-78. The leading cause of death in the entire age group is injuries, including automobile accidents.

With the two forms of violent death surpassing all other causes of death in the 15-24 age group, CDC has launched a study aimed at identifying likely victims and finding ways to prevent violent deaths.

"These are important and preventable public health problems," said Dr. Mark Rosenberg, director of the CDC's new violence epidemiology branch.