Recent studies have indicated that drug use is not only correlated with, but is also highly predictive of recidivism by particular defendants. In view of this relationship, it is essential to ensure that criminal justice decisions are based on reliable information about defendants' drug use. This study used urinalysis test results on arrestees obtained by the District of Columbia, the only large jurisdiction where virtually all arrestees are routinely tested by urinalysis, and PROMIS records to construct two data files: a cross-sectional file which merged prosecutor case information and the urinalysis results for almost 60,000 arrestees processed in D.C. Superior Court from 1973 through 1977 and a longitudinal file containing complete records on 7,087 randomly selected arrestees from 1973 through 1978. Drugs most frequently found in the urinalysis specimens were morphine (heroin is metabolized rapidly in the body to morphine), phenmetrazine, methadone, and quinine. Persons found to be using drugs were more likely to be rearrested than persons who never had a positive test. Drug users were most likely to be charged with larceny offenses, followed by burglary, drug offenses, robbery, and bail violations. More than 80 percent were not charged with any drug offense at all. Drug users' arrest rates for larcenies, burglaries, drug offenses, robberies, and bail violations were all two to three times greater than those of nonusers, although the two groups' arrest rates for violent crimes were similar. Data on arrestees' activities after their first arrest showed that persons with a positive urine test were more likely to be active criminals than nonusers, even after controlling for age and arrest history. The paper explores the implications of these findings for criminal justice policy. Tables and 25 footnotes are included.