Adverse drug reactions (ADR) are an important clinical problem and their impact on the Spanish population is unknown. The aim of this study was to provide an overview of ADR based on a review of observational studies published in Spain since 1974. Twenty-seven studies of Spanish patients were selected from a total of 90 articles published in national and international journals between 1974 and 2004. Studies eligible for inclusion were identified with an especially designed checklist. The incidence of ADR among 139,799 patients was 3%; 0.6% of the events led to death, and 17% led to severe illness. Two-thirds (67%) of the ADR were classified as type A. The rate of hospitalization for ADR was 6.5%, and mean length stay was 4 +/- 1.3 days. Thirteen percent of the ADR were considered confirmed, and 57% were considered probable. Half (50%) of the ADR were considered preventable. The groups of medications involved most frequently were antibacterial, NSAID, psychoactive medications, bronchodilators, digoxin, non-opiate analgesics, vasodilators, antidiabetics and loop diuretics. The organs and systems involved most frequently were the digestive tract, skin, central nervous system, cardiovascular, endocrine and respiratory systems. The factors associated most strongly with ADR were advanced age and use of multiple medications. The relationships of gender, previous history of ADR and comorbidity with the incidence of ADR have yet to be established. This review of ADR provides useful information on the incidence of ADR in the Spanish population and serves as a baseline for future studies.