The Navy's "zero-tolerance" drug policy has been in effect since 1981. Since then, the Navy has pursued an aggressive urinalysis drug-testing program. The objectives of this testing program have been to deter and detect drug abuse as well as to provide data on the prevalence of drug abuse in the Navy. All uniformed personnel are subject to random urinalysis testing on a continuing basis. Drug use in the Navy was compared to comparable groups of civilians over time in order to determine the impact of testing. Deterrence was defined as the proportion of drug users in the Navy (individuals who would use drugs at least once during a 30-day period) who no longer would use drugs if testing were conducted at a specific monthly test rate (e.g., 20 percent). Estimates of the deterrent effect of drug testing assume that the difference in drug use rates between Navy personnel and a demographically comparable group of civilians can be attributed almost entirely to drug testing. Evidence from the 1992 Worldwide Survey of Substance Abuse and Health Behaviors Among Military Personnel (Bray et al., 1992) supports this assumption. Findings show that the Navy's drug testing at current levels deters almost 60 percent of potential drug use. Thus, drug testing apparently is an effective technique for reducing drug use in the Navy. 2 figures, 3 tables, and 13 references