Like many other drugs co-trimoxazole can induce a large number of different skin reactions, mainly of allergic pathogenesis. The majority of these reactions, such as urticarial, purpuric, maculo-papular, and pustular exanthemas as well as photallergic reactions, generally do not endanger the life of the patient. Apart from very rare cases of anaphylactic shock, there is a risk of lethality associated with Stevens-Johnson syndrome and Lyell's syndrome of approximately 1% and 30%, respectively. Between 1981 and 1985, an extensive epidemiological survey was carried out in Germany which enabled approximation of drug induced severe skin reactions (Lyell's syndrome, Stevens-Johnson syndrome). Preliminary evaluation of the survey allowed the identification of 217 Lyell's syndromes and 296 Stevens-Johnson syndromes. The total registration rate resulting from an almost complete survey of all applicable medical units (dermatology, burns, intensive care) was 92%. The basic risk amongst the population of acquiring Lyell's syndrome and Stevens-Johnson syndrome, as calculated from the data gathered between 1981 and 1985, is 0.8 and 1.0 per year, and per one million inhabitants respectively, in the drug-related incidence calculation. Co-trimoxazole is in the upper third of the table of drugs which certainly, probably, or possibly induced a Lyell's syndrome and in the middle of the table of those that induced Stevens-Johnson syndrome.