We examined more than 1,400 dermatologic patients with clinically defined (but having unknown or presumably multiple etiology) affections. The investigation revealed the presence of antitoxoplasma antibodies in more than 50% of the patients, but in only 11% of the cases did the serological analyses give evidence of an active form of disease. It was possible to prove the toxoplasmic etiology of 29 cases of chronic prurigo and of 4 cases of dermatocellulitis. The same infection was involved in a few cases of different dermatoses and in two cases of dermatomyositis-like syndrome. Pseudotumoral granulomatous localizations occurred in immunosuppressed patients. We suggest an 'immunological key' to explain the polymorphism of the cutaneous manifestations. The practical interest of this new knowledge and its importance as a field of interdisciplinary studies are emphasized.