The clinical, radiologic, ultrastructural, and histopathologic findings in 14 patients with systemic mastocytosis were evaluated. Seven patients had evidence of urticaria pigmentosa (UP) and seven patients presented with no recognizable cutaneous lesions. There were no major clinical differences between patients with or without UP except for splenomegaly, which was present in one/seven patients with UP and five/seven patients without UP and the median age, 44 in patients with UP, and 75 in patients without UP. Bone marrow involvement was present in 13/13 specimens studied. Involvement was both focal and diffuse. The focal involvement occurred frequently in a perivascular and paratrabecular location. The diffuse involvement resembled myelofibrosis. Involved lymph nodes exhibited prominent sinusoidal and paracortical infiltration by mast cells. Splenic involvement was characterized by fibrosis occurring both focally and diffusely. The focal splenic involvement was perivascular and involved both the red and white pulp in a nonpreferential manner. Liver specimens showed prominent portal fibrosis. The morphology of the mast cells in the different lesions varied considerably; some were typical, others were spindle-shaped, and some resembled histocytes. The mast cells reacted positively with toluidine blue and chloroacetate esterase. Six patients had radiologic changes: three were osteoblastic, two osteolytic, and one osteoblastic and osteolytic. Two patients developed a poorly differentiated lymphoreticular tumor and one a myeloproliferative disorder after the diagnosis of mastocytosis.