Unwanted drug reactions (UDR) in routine clinical practice remain largely unrecorded, even though such documentation can significantly contribute to quality assurance in drug therapy. This prospective study was undertaken to ascertain within a defined time period the frequency and significance of UDR in the medical wards of a general hospital (focus on gastroenterology) and find the organisational structures needed for its documentation. All observed UDRs among a total of 4032 in-patients were recorded on a simple registration form, filled in by doctors assigned to collect the information, which was then analysed in cooperation with the Drug Commission of the German Medical Council (AKdA). UDRs were recorded in 315 patients (7.8%), 135 of them (43%) already present on admission. 101 patients had been admitted because of the UDR. Gastrointestinal disorders, particularly peptic ulcer, and skin rash were the most common UDRs. Individual drugs most frequently responsible were acetylsalicylic acid, cyclophosphamide and digoxin, the most common drug groups were nonsteroid analgesics, antibiotics and cytostatic drugs. The method employed proved to be practicable and cost-effective. A simple and cost-effective system, integrated into the daily medical routine, can heighten awareness of UDRs among doctors and thus improve drug safety in hospitals, while concomitantly providing an important feature of further medical education.