Proliferation of tissue mast cells in more than one organ is a rare disease, called generalized mastocytosis. Findings obtained in 35 cases were compared with 125 case reports in order to elucidate the course of the disease and its clinical picture. The results indicated that generalized mastocytosis has to be divided into two variants, designated systemic mastocytosis and malignant mastocytosis. Systemic mastocytosis is characterized by urticaria pigmentosa-like skin eruptions, with simultaneous infiltration of at least one visceral organ (usually bone marrow). Other frequently involved organs are spleen, liver, and lymph nodes. The age curve is biphasic, with one peak in early childhood and another in the 6th decade. The prognosis of systemic mastocytosis is generally favorable (actuarial survival 0.88 one year after diagnosis). In contrast to systemic mastocytosis, malignant mastocytosis does not show urticaria pigmentosa-like skin lesions. In this variant of generalized mastocytosis, frequently involved organs are bone marrow, spleen, liver, and lymph nodes. Common symptoms are anemia and eosinophilia. The age curve shows a peak in later life (6th and 7th decades). Malignant mastocytosis is never seen in children.(ABSTRACT TRUNCATED AT 250 WORDS)