The 1999 Annual Report on Drug Use Among Adult and Juvenile Arrestees provides an overview of 1999 Arrestee Drug Abuse Monitoring (ADAM) Program findings and detailed site-by-site tables on drug use among adult and juvenile arrestees. This NIJ Research Report includes data collected from more than 30,000 adult male arrestees in 34 sites, 10,000 female arrestees in 32 sites, 2,500 male juvenile detainees in 9 sites, and 400 female juvenile detainees in 6 sites. (This is the first ADAM annual report to include data on drug use by female juvenile detainees.) When comparing data with results from the previous year, the median rate for drug use remained the same in adult males, while the median rate for adult females increased 3 percent. Marijuana was the most frequent drug detected among adult males, and cocaine was most frequently detected in adult females. Marijuana use was more than 6 times higher than cocaine use for both juvenile males and females. Significant differences in drug use were found city by city, indicating the importance of tracking patterns on a local level to be able to tailor drug-control approaches to local needs.