Abstract: To assess the quality and relevance of adverse drug reactions (ADRs) published as Letters to the Editor (LE) in Spanish medical journals. Observational study. LE on adverse drug reactions published over 5 years (1994-98). Four Spanish medical journals (Medicina Clínica, Revista Clínica Española, Atención Primaria and Anales de Medicina Interna). Patient characteristics, drugs, ADR, causality algorithm, minimum criteria, and publication relevance. Out of 2244 LE, 204 (9.1%) reported ADRs, which included 235 cases. The therapeutic subgroups most commonly implicated were anticoagulants and antiplatelet drugs, antibiotics, and antineoplastic agents; 20.4% of the drugs were recently marketed. ADRs most commonly involved the nervous system (13.6%), liver (10.2%), skin and appendages (9.8%), general reactions (9.8%), and the digestive system (8.1%). The reactions were moderate in 50.2% of cases and severe/fatal in 34%. The mean causality algorithm value (5.9+/-2.2) was similar among journals. Of the ADRs, 28 (11.9%) were definitive, 182 (77%) possible or probable, and 26 (11.1%) improbable or conditional; 10.2% were unknown. There were no differences in the mean minimum publication criteria (9.5+/-1.2). Publication relevance was 3.2+/-1.6 points, and higher in Medicina Clínica. ADRs constitute an important part of LE in the journals studied. The causal relationship is acceptable, the documentation quality is high, with few unknown reactions and ADRs to recently marketed drugs. Relevance is generally low, although greater in Medicina Clínica.