A 14-month (1992/3) prospective study was performed in two departments of the University Hospital Centre (UHC) in Zagreb. The aim of the study was to assess the rate of drug-related hospitalizations, drugs that caused adverse drug reactions (ADRs), and all factors which could have been of importance for their appearance. One hundred and thirty (2.5%) of 5,227 patients were admitted to hospital because of ADRs. The most frequently ADR-related drugs were nonsteroidal anti-inflammatory drugs and analgesics (64.6%). They were followed by cardiovascular agents (20.8%) and antimicrobials (3.8%). Acetylsalicylic acid (aspirin) caused 38.5% of hospital admissions, other nonsteroidal anti-inflammatory drugs (NSAIDs) 23.1% and medigoxin 15.4% of hospitalizations. The most frequent ADRs were upper gastro-intestinal tract bleeding (64.6%), cardiac rhythm disturbances (13.9%), blood cell disorders (4.6%) and hypoglycemia (2.3%). Regarding the patients' age, 52.3% of patients was younger and 47.7% older than 65. Sixty-one point five percent of patients was taking more than one drug, older patients (48 patients--77.4%) have been taking a significantly higher number of drugs than the younger (32 patients--47.1%) (p < 0.0001) ones. Drug interactions caused 23.8% of ADRs. Only 11 (8.5%) of patients suspected themselves that the drug might have caused the ADR. Improvement was observed in the majority of patients (65.4%), 25.4% recovered completely, 4 (3.0%) died in the hospital because of ADRs. 3.0% of patients as well died of their underlying diseases, 2.3% were transferred to other departments for their underlying diseases, and one patient left the hospital on his free will.