<i>Background:</i> Patients with systemic mastocytosis have increased numbers of mast cells in the bone marrow and other organs, such as the liver, spleen, gastrointestinal tract and skin. Symptoms result from the local and remote effects of mediator release from mast cells and from the local effects of increased mast cell numbers in various organs. Patients with mast cell activation experience many of the same clinical symptoms as do patients with systemic mastocytosis from chronic or spontaneous release of mast cell mediators. We report 4 patients with mast cell activation symptoms from selective release of prostaglandin (PG) D₂, but not histamine, and their improvement with aspirin therapy. <i>Methods:</i> Bone marrow biopsy specimens obtained from 4 patients with symptoms suggestive of mastocytosis were examined by tryptase immunostaining. Baseline levels of serum tryptase and urinary 11β-PGF₂_α and N-methylhistamine were obtained. In 2 of the 4 patients, urinary 11β-PGF₂_α and N-methylhistamine samples were also measured during acute symptoms. <i>Results:</i> Baseline increase in urinary excretion of the PGD₂ metabolite 11β-PGF₂_α was found in 2 patients. In the remaining 2 patients, baseline levels of urinary 11β-PGF₂_α and N-methylhistamine were normal, but during acute symptoms, the excretion of 11β-PGF₂_α increased markedly. Treatment with aspirin resulted in normalization of 11β-PGF₂_α excretion in the 2 patients with elevated baseline levels and in prevention of symptoms in all 4 patients. <i>Conclusions:</i> These results suggest that mast cell activation may be manifested by a selective excessive release of PGD₂. These patients respond to administration of aspirin but not to antihistamines.