Adverse drug reactions represent a major source of morbidity in hospitalized- and outpatients. Adverse drug reactions may affect the function of any organ or system. Due to their clinical nonspecificity adverse drug reactions are often difficult to be recognized. The clinical manifestation, the type and severity of adverse drug reactions vary a great deal depending on patient- and drug-related factors. In recent years growing evidence has emerged implicating involvement of free radicals and reactive oxygen species in chronic degenerative diseases and in acute conditions, such as stroke and infection. The review focuses on the role of reactive oxygen species in the pathogenesis of drug-induced disease with special reference to hepatotoxicity, nephrotoxicity and allergic disease. The participation of reactive oxygen species in the development of adverse drug reactions induced by special drug classes (non-steroidal anti-inflammatory drugs, anti-infective agents, cytostatics) is described. Results from studies on animals and humans reveal the putative implication of oxidative stress in drug-induced disease states.