The reports from the emergency service of La Paz General Hospital were daily reviewed for 4 months to investigate the number of consultations which, on the judgement of the physician on care, were due to adverse reactions to drugs. An overall number of 11,326 patients consulted. In 438 (3.9%) it was considered that the consultation was due to one or more definite, likely or possible adverse drug reactions. In 69 patients (15.8%), the reactions were considered to be severe, and 54 (12.3%) required admission; 59 reactions (13.4%) were moderate, and 310 (70.8%) were mild. The most common localizations were the skin and its appendages (37.7%) and gastrointestinal tract (25.3%). The most commonly implicated pharmacologic groups were analgesic and non-steroidal anti-inflammatory drugs (33.6%) and antimicrobials (22.1%). The incidence of adverse reactions was significantly higher in women (4.4% vs 3.3% in males, p less than 0.01). Depending on the age groups (14-29, 30-59, greater than or equal to 60 years) the incidence of nonallergic adverse reactions was significantly higher in patients aged 60 years or more (1.5%, 2%, 2.9%; chi 2 = 15.2, gl = 2, p less than 0.001). In presumably allergic adverse reactions, the incidence was significantly higher among those under 30 years (2.9%, 2.2%, 0.5%; chi 2 = 50.2; gl = 2, p less than 0.0001). The incidence of severe adverse reactions was significantly higher in patients over age 60 years (0.2%, 0.6%, 1.2%, chi 2 = 29.2, gl = 2, p less than 0.001). In 32% of cases the adverse reactions might have been prevented.