Based on (1) the large numbers of mast cells present in neurofibromas, (2) the possibility that these mast cells contribute directly to neurofibroma growth, and (3) the ability of ketotifen therapy to stabilize (ie, block) mast-cell secretion, treatment with ketotifen was started in a patient with severe neurofibromatosis (NF) in August 1983. Subsequently, ten additional patients with one or more symptomatic neurofibromas were treated with comparable doses of ketotifen, 2 to 4 mg/d, orally administered for 30 to 43 months. This represents a total of 389 patient-months or 32.4 patient-years. All of these patients showed an unequivocal decrease in neurofibroma-associated pruritus and/or pain and tenderness; a consistent, but less-uniform, decrease in the rate of neurofibroma growth; and an unexpected improvement in overall sense of well-being, productivity, and general performance. It appears likely that mast-cell secretions do contribute to the growth and associated symptoms of neurofibromas, and that mast-cell blockers, such as ketotifen therapy, can retard this growth.