A 64-year-old man had urticaria pigmentosa and myelodysplasia (refractory anemia with excess blast cells; partial trisomy 8 syndrome) without increased numbers of marrow mast cells. Clonal marrow assays in agar demonstrated normal to increased colony-forming units of granulocytes/macrophages. In long-term liquid cultures containing mast cell growth factor (interleukin 3), his marrow cells proliferated after 3 weeks to produce abnormal myeloid precursors similar to those in the corresponding marrow aspirate specimen. Cells with basophilic-staining granules were less abundant in comparison with normal marrow specimens cultured similarly. These results suggest that the mast cells in this patient are not of the same clone as the preleukemic marrow cells, although the possible marrow-cell origin of urticaria pigmentosa mast cells cannot be excluded. Previous reports suggest that urticaria pigmentosa without systemic mastocytosis occurs as a nonspecific abnormality in a variety of myeloid, lymphoid, and nonhematologic malignancies. Our data also support this hypothesis that urticaria pigmentosa is a reactive process rather than a manifestation of clonal proliferation of the primary malignancy.