OBJECTIVE To assess the occurrence and pattern of adverse drug reactions as a cause for acute hospital admission. METHODS In 681 randomly selected patients, acutely admitted to a clinic of internal medicine at a Swedish university hospital, information was collected from their medical records about current symptoms and use of drugs, previous diseases and the results of medical investigations and tests. In addition, a standardized interview according to a questionnaire was carried out. A group of experts in clinical pharmacology assessed the data obtained from the patients' case records and the results of the interviews, and then, according to WHO criteria, judged the probability that an adverse drug reaction could have caused or contributed to the actual admission to hospital. RESULTS Out of the 681 cases included, 94 (13.8%) had symptoms and signs that were judged as drug-related and that had caused or contributed to the admission. Eighty-two patients (12.0%) had altogether 99 symptoms that were classified as adverse drug reactions. Of these, 91% were type A reactions. The relationship between the medication and the reaction was judged certain in eight, probable in 17, and possible in 74 cases. The most common adverse drug reactions were cardiovascular (36.3%). Twelve patients (1.8%) had symptoms indicating intoxications. CONCLUSIONS The prevalence of drug-related problems causing or contributing to admission to a clinic of internal medicine is high and is dominated by type A reactions, i.e. reactions in principle predictable and preventable. This implies a possibility to increase drug safety by preventive measures.