A woman had the typical clinical, cutaneous, and muscular picture of dermatomyositis but without some of the biochemical alterations of that disease. The positive serological investigations for toxoplasmosis, the pressure in muscles of very numerous toxoplasms and their culture in mice, and the improvement after treatment with pyrimethamine and sulphametoxypyridine demonstrated that the signs of dermatomyositis had been caused by infection with toxoplasmosis.