The ultrastructural aspects of mast cells from skin lesions in 2 patients with mastocytoma were compared to those from lesions in 4 patients with mastocytosis, 3 with cutaneous and 1 with systemic involvement. In mastocytoma, the mast cells accumulated in a large mass. They revealed short cytoplasmic villi and were similar, morphologically, to mast cells of normal skin and gingiva. In the diffuse types of the disease, cutaneous and systemic, the cells were arranged in small groups. In addition to normal mast cells, there were also irregular or bizarre-shaped cells presenting long and twisted cytoplasmic protrusions. In the case with the systemic involvement and in 2 of 3 cases with apparently cutaneous diffuse lesions, the villi of adjacent cells interlaced and showed a tendency to form mast cell aggregates. The pathognomonic value of the above observation seems worthy of further investigations.