Insomnia is a frequent health problem in prison, but little is known about its severity and duration. The objective was to find out whether subjective sleep quality improves during time and which factors influence improvement. Fifty-two randomly chosen prisoners complaining of insomnia at the Geneva remand prison were interviewed (T1) and followed up ten days (T2) and two months (T3) later. They received hypnotics habitually prescribed by prison physicians (benzodiazepines, chloralhydrate, zolpidem). After ten days 40 patients were still in prison and agreed to participate and after two months 16 prisoners could be re-evaluated. Total Pittsburgh Sleep Quality Index (PSQI) scores at T1 were 12.3 +/- 4.7. At T2 and T3, PSQI scores improved significantly, whereas General Health Questionnaire (GHQ) scores and conditions of imprisonment were similar. Only 12.5% of patients at T2, and 6.25% at T3, were 'good sleepers' (PSQI scores =/< 5). Cocaine users and prisoners with a healthy lifestyle reported the greatest improvement of PSQI scores. Our study shows that significant improvement of PSQI scores takes place in the first one to two weeks. However, in spite of regular drug treatment during the following weeks, PSQI scores persisted at a clinically significant level two months later. Drug treatment in prison only partially improves insomnia.