The authors report the case of a patient suffering from systemic mastocytosis with cutaneous, medullary, bone, hepatic and splenic involvement, the clinical and haematological picture of which was that of myeloid splenomegaly. Furthermore, this picture included hypersplenism, clearly demonstrated by isotopic study and the successful results of splenectomy. Blood histamine levels were normal though urinary excretion of histamine was increased. Histamine levels in the spleen were greatly increased. Serotonin levels in the blood and its urinary metabolites were normal. Heparin activity was demonstrated in the spleen, but not in the blood.