To determine the effect of hospitalization on errors in patients' drug regimens, 157 consecutive patients discharged on regimens of four or more drugs were identified, of whom 94 were eligible for study. Thirty-four (79%) of 43 local patients were interviewed within 1 month after discharge, and 28 (55%) of 51 patients who were mailed a questionnaire responded. Regimen errors were detected in 50% of both groups. Overall, 20 (32%) of 62 patients had incorrectly added or deleted a drug, and 11 (18%) were taking the correct drugs but had errors in dosing. Twelve potentially serious errors were detected. Patients with regimen errors had been discharged taking more drugs (6.1 vs 5.10) and tended to have more drug changes during hospitalization (2.7 vs 1.90). Particular attention should be paid to drug regimens during hospitalization and in subsequent follow-up visits.