573 elderly patients admitted to the general medical and geriatric beds of a teaching hospital in January, 1983, were receiving an average of 2.14 drugs on admission, 5.48 during inpatient stay, and 3.47 on discharge. Contraindicated or adversely interacting drugs were identified in 200 (3.2%) of 6160 prescriptions. 136 (23.7%) patients were affected. 7 patients received drugs to which they had had adverse reactions. There were a further 60 contraindicated and 133 adversely interacting drugs. 117 prescriptions were potentially hazardous and another 27 interactions may have led to suboptimal treatment. 131 (65.6%) undesirable prescriptions were deemed avoidable and a further 36 (18%) were probably so. The frequency of errors was higher in admission medication (5.3%) than in hospital prescriptions (2.9%). The importance, origin, and prevention of prescribing errors in drug-induced morbidity in the elderly are discussed.