The diagnosis of systemic mastocytosis without urticaria pigmentosa has been made with increasing frequency since modern methods of histamine assay have been used clinically. We examined the incidence of urticaria-angioedema and mastocytosis over a recent 12-month period. Of 490 new patients we saw, 52 had urticaria-angioedema, and ten had evidence of excess histamine +/- PGD2, with at least ten mast cells per high-power field on skin biopsy. The average age was approximately 35 years; the male:female ratio was 1:4 for urticaria-angioedema and 1:2 for mastocytosis. Symptoms of mastocytosis included flushing, abdominal cramping/diarrhea, syncope, urticaria-angioedema, pruritus, and headache. Symptoms have typically been prevented by a combination of H1 and H2 antagonists, with addition of a cyclo-oxygenase inhibitor in syncopal cases. Acute hypotension has responded to epinephrine.