To describe the frequency and pattern of drug-related morbidity that results in hospital admission and the extent to which these admissions are avoidable, we prospectively reviewed the charts of 452 consecutive patients admitted to the intensive care unit or internal medicine service of a university-affiliated, county hospital. Of these, 73 (16.2%) were admitted due to drug-related morbidity. Forty patients (54.8%) experienced drug therapy failure, 24 (32.9%) had an adverse reaction, and 9 (12.3%) had overdoses. Thirty-six (49.3%) of these admissions were definitely avoidable. Compared with patients admitted for other reasons, these patients were more likely to report noncompliance with drug therapy (65.8% vs 15.7%, p < 0.0001) and were taking more drugs (p = 0.0037). We conclude that approximately half of drug-related hospital admissions are avoidable. Targeting patients taking several drugs and with a history of noncompliance may reduce this problem.