Six patients had hematologic malignancies and coincident urticaria pigmentosa, five with the disseminated maculopapular form and one with the plaque form. Two patients had the juvenile-onset variety; the remainder had the adult eruptive variety. None of the patients complained of symptoms that could be attributed to liberation of histamine. In the two patients with juvenile-onset urticaria pigmentosa, the hematologic malignancies developed at the age of 17 years; one had Hodgkin's disease, and the other had acute myelomonocytic leukemia. In three patients with adult eruptive urticaria pigmentosa, the cutaneous lesions developed within 12 months of the diagnoses of lymphocytic lymphoma (two patients) and evolving myelomonocytic leukemia (one patient). In the remaining patient, cutaneous lesions developed many years before chronic lymphocytic leukemia was diagnosed. None of the patients had systemic mastocytosis. Skin biopsy specimens from all six patients showed an increase in dermal and perivascular round cells, and mast cells were seen in specimens from five of the six patients. In patients who received cytotoxic drugs for the hematologic malignancy, there was no change in the urticaria pigmentosa.