URTICARIA pigmentosa of the large nodular type is of sufficient rarity to cause diagnostic problems. The condition may remain undiagnosed unless a specimen is removed and examined microscopically after the tissue has been stained with one of the basic aniline dyes. Finnerud1and Little2in excellent reviews stated that in approximately 10 per cent of cases of urticaria pigmentosa the lesions are of the nodular type. However, the occurrence of large tumors in urticaria pigmentosa similar to those observed in our case is apparently unrecognized. We wish to report such a case and to comment briefly on the prognosis and the association of heparin with the mast cells. REPORT OF A CASE A 21 month old American boy of Syrian parentage was seen at the Mayo Clinic in November 1946. Since birth this patient had been normal except for brown oval and round nodules on the neck,