Since about 1950 especially, dermatologists world-wide have been utilizing the positive side-effects, discovered by chance, of all groups of antibiotic and antimicrobial drugs. These drugs are used to treat certain non-microbially induced dermatoses, without any knowledge of the mechanisms involved. A short history is given and the most important drugs and the indications for their use are described. The following drugs are undoubtedly effective and sometimes even the therapy of choice: tetracyclines in acne vulgaris and rosacea (including rosacea keratitis); penicillin G in acrodermatitis atrophicans and cold urticaria; dapsone in dermatitis herpetiformis and - as a powerful adjuvant - in acne vulgaris and rosacea. Before the discovery of the socalled immunodepressive drugs, tetracycline was the only alternative to - or at least a highly effective adjuvant of - cortisone in dermatomyositis and chloroquine in localised and systemic lupus erythematosus. Finally, clioquinole was life-saving in acrodermatitis continua in children until this condition was recently identified as a zinc-deficiency syndrome. Therapeutical mechanisms have been found only in the case of acne, rosacea and dermatitis herpetiformis. In most other diseases the nature of the therapeutical effectiveness of antibiotic and antimicrobial drugs still remains a mystery.