To determine trends in gang homicides and the population at greatest risk for homicide by reviewing all gang-related homicides in Los Angeles County, California, from January 1979 to December 1994. Homicide files of the Los Angeles Police Department and the Los Angeles County Sheriff's Department from 1979 to 1994 involving violent street gang activity were reviewed. Gang files were reviewed for demographic data, weapons used, homicides by drive-by shootings, and times and geographic areas of occurrence. Los Angeles County from January 1, 1979, to December 31, 1994. Age, race, and sex of gang-related homicide victims; frequency of weapon use; and the change in gang-related homicide rates during the study period. A total of 7288 gang-related homicides occurred in Los Angeles County from 1979 through 1994; 5541 of these homicides occurred in Los Angeles Police Department and Los Angeles County Sheriff's Department jurisdictions. During the study period, the proportion of all homicides that were gang related increased from 18.1% to 43.0% (P < .001). Of the 5541 gang-related homicide victims, 4580 (85.6%) were aged 15 to 34 years, 93.3% were African American or Hispanic, 5157 (93.2%) were male, 3559 (64.2%) were gang members, and 1408 (25.4%) occurred during drive-by shootings. Firearms were used in an increasing proportion of homicides, from 71.4% in 1979 to 94.5% in 1994. Homicides by semiautomatic handguns dramatically increased during the study period. Gang-related homicide rates for African-American males aged 15 to 19 years increased from 60.50 per 100,000 population per year in 1979 to 1981 to 192.41 per 100,000 population per year in 1989 to 1991. Gang-related homicides in Los Angeles County have reached epidemic proportions and are a major public health problem. To prevent gang violence, the root causes of violent street gang formation must be alleviated, the cycle of violent street gang involvement must be broken, and access to firearms must be limited.