426 consecutive patients admitted to a Danish University Department of Cardiology have been studied. Drug intake prior to admission by each patient was ascertained from medical records and personal interviews. Adverse drug reactions (ADR) were the primary cause of admission in 49 patients (11.5%), and 16 patients (3.8%) were admitted due to drug non-compliance (DNC). Thiazide diuretics, beta-adrenoceptor blocking agents and calcium antagonists accounted for almost 60% of all the ADR-related admissions. Patients admitted for ADR took significantly more drugs than patients admitted for other reasons. DNC was not correlated with the number of prescribed drugs. It is concluded that drug-related hospital admissions are an important medical and economic problem. Most of the ADRs were well-known and predictable actions of the drugs, and could have been avoided by more careful clinical and laboratory monitoring of the patients. Most of the DNC, too, could have been avoided by giving better information to the patients.