The model for the crack epidemic consists of four phases: incubation, expansion, plateau, and decline. Each successive phase can be distinguished by the quantity and type of persons first initiating crack use and continuing its use over time. Data to test the model were obtained from Manhattan's Drug Use Forecasting program. This program conducts voluntary interviews with arrestees regarding their drug abuse and also performs voluntary urine drug testing on arrestees. Manhattan's Drug Use Forecasting program began in the second quarter of 1987, and at the time of this analysis, data were available through the first quarter of 1993, a period of 6 years. This sample includes a disproportionately serious group of offenders. Data from arrestees interviewed in the Drug Use Forecasting program suggest that crack popularity in New York City expanded rapidly from 1981 to 1985 among existing hard drug users. Crack was subsequently popular from 1984 to 1987 among youths first coming of age and experimenting with hard drugs (about age 18), the plateau phase. Starting in 1988, many fewer high-risk youths initiated crack use, a decline phase. These findings suggest that the crack problem facing New York City in the 1990's results from existing crack users persisting in their habits. 14 references and 4 tables