Aims. To describe self-reported levels of substance misuse before arrest among remanded prisoners (unconvicted prisoners awaiting trial), to assess their degree of dependency on opiates and stimulants and to record their experiences of treatment in prison. Design. Random selection of subjects from prisons chosen to give a geographical spread across England and Wales; self-report at semi-structured interview, plus examination of the prison medical record. Setting. Thirteen male prisons, three Young Offenders' Institutions and three womens' prisons. Participants. Nine hundred and ninety-five consenting, unconvicted prisoners, randomly selected from all locations within the prisons: 750 men (9.4% sample) and 245 women (82.2% of all remanded women). Measurements. CAGE Questionnaire, Severity of Dependence Scales (SDS) for daily users of opiates and/or stimulants. Findings. Before arrest, 145 (19.3%) men and 72 (29.4%) women had been dependent on street drugs; 91 (12.1%) men and 16 (6.5%) women were solely dependent on alcohol. Seventeen (2.3%) men and four (1.6%) women reported injecting drugs during this imprisonment. Mean SDS scores were 10.6 for opiate and 7.7 for stimulant users. 244 (25%) of all subjects described withdrawal symptoms on reception into custody; 157 (16%) reported being prescribed some symptomatic relief; 235 (24%) requested treatment at interview. Conclusions. By extrapolation, 1905 people-23% of all unconvicted prisoners-want treatment for substance misuse. This apparent shortfall in provision must be addressed; the rapidity with which remanded prisoners return to the community dictates that prison and community services should be closely linked.