Background: Adverse drug reactions (ADRs) are a threat to patients' health and quality of life, and can generate significant expenses. They are generally underreported, with different rates in different health care systems. Methods: We conducted a 6-month survey of all primary admissions to the medical emergency department of a university hospital and assessed the rate, characteristics, avoidability, and marginal costs of ADRs. Results: A total of 7% of all admissions were mainly caused by ADRs. The most frequent were gastrointestinal bleeding (22.3%) and febrile neutropenia (14.4%). Anticancer drugs were involved in 22.7% of the cases, and anticoagulants, analgesics, and non-steroidal anti-inflammatory drugs in 8% each. Physicians had prescribed 70% of these drugs. Patients were predominantly treated in intermediate care units and ordinary wards. The mean cost per case amounted to CHF 3586+/-342, or a total of CHF 821204 over the 6-month-period (1 CHF=0.56 US$=0.87 Euro). A total of 67% were considered definitely imputable to drug effects and 32% were retrospectively regarded as avoidable. Conclusions: Interventions aimed at reducing the incidence of ADRs should be directed towards both patient education and physician training. This could save hospitals admissions and money, and could be used as an indicator of prescription quality.