A patient with urticaria pigmentosa and systemic mastocytosis developed hypotension following indomethacin administration. He then developed further episodes not related to indomethacin. Based upon the experience of others with the management of patients with systemic mastocytosis who showed exceptional reaction to cyclooxygenase inhibition, it was decided to treat him with H1 and H2 blockade followed by aspirin, another cyclooxygenase inhibitor. The procedure was carried out under careful observation with cardiac monitoring. After 160 mg of aspirin, he developed hypotension, tachycardia, and flushing accompanied by difficulty of breathing and headache. A vasoconstrictor drug (levarterenol) was administered. The patient's symptoms subsided, and after 1 hour aspirin was again administered, this time with no side effects. The dosage was increased to 975 mg every 6 hours, and he has had no further hypotensive episodes on this regime for 2 years. Cyclooxygenase inhibition, combined with H1 and H2 blockade, is an effective treatment for this condition, but for these patients initiation of aspirin therapy should be carried out with extreme care.