Since 1996, there has been a program of mandatory drug testing (MDT) using urinalysis in all prisons in the United Kingdom. The majority of all drug testing in prisons is done under the random mandatory drug testing (RMDT) program in which a fixed proportion of inmates are randomly chosen for testing each month. The Home Office has directed the study of the current operation and impact of the MDT program. The study addresses four main questions: (1) to what does RMDT data provide an indication of the pattern of drug misuse in prisons; (2) is it possible to extrapolate from RMDT results to estimate a measure of drug use; (3) what levels of testing are necessary to provide significant results for individual establishments, prison service regions, and nationally; and (4) is there evidence that the MDT program influences drug use in prisons? Study methodology included a survey of prisoners regarding their drug use in prisons and their experiences of prevention measures. Qualitative interviews were conducted with prison staff regarding the administration of the MDT program and secondary data analysis was conducted on the prevalence of drug use in prisons. Survey data indicated that 39 percent of inmates had used an illicit drug at some time during their current prison stay. Another 67 percent of inmates reported participation in some form of the MDT program. RMDT data showed fewer positive results than the self-report survey data indicated should be the case. However, the analysis suggested that the national level RMDT rates provided a reasonable indication of levels of prison drug use, particularly for cannabis. The majority of inmates reported they were extremely unlikely to use drugs during their current or future prison stays. These findings suggest that the MDT program is effectively discouraging drug use in prison. Tables, figures, appendixes