['Adverse drug events (ADE) contribute significantly to hospital admissions. Prospective New Zealand data are scant, and the ability of clinical coding to identify ADE associated admissions is uncertain. Outcomes after cessation of causative medications are unknown.', 'To assess the frequency, nature and causality of ADE associated with acute admissions to General Medicine at Christchurch Hospital.', 'Prospective observational study of patients admitted to our medical team over 20 weeks.', 'Of 336 admissions, 96 (28.6%) were ADE related. Sixty-five (19.3%) were caused by an ADE, and 31 (9.2%) were contributed to by an ADE. The mean age of non-ADE patients was 64.3 years (range 16-91), which was similar to the mean age of ADE patients (65.9 years; 21-92). However, if intentional overdoses and recreational drug use were excluded, ADE patients were significantly older at 72.4 years (21-92) (P = 0.0007). ADE patients took more regular medications on admission (mean 6.6, range 0-22) than non-ADE patients (mean 5.0, 0-18), (P = 0.003). The average length of stay was similar. The commonest medications implicated were vasodilators, psychotropics and diuretics. The most common adverse effects were postural hypotension and/or vasovagal syncope (29% of ADE), intentional overdoses and recreational drug use (15%) and acute renal failure and/or clinical dehydration (10%). Seventy-six patients had culprit medications stopped or reduced, and this potentially contributed to six readmissions. Coding identified 61% of ADE associated admissions.'] Are adverse drug events a major cause of acute medical admission?