A double-blind crossover study of the efficacy of disodium cromoglycate given by mouth to control the cutaneous, gastrointestinal and central-nervous-system manifestations of systemic mastocytosis was carried out in five patients for periods of eight to 32 months. In 15 of 18 trials, disodium cromoglycate produced marked amelioration of the clinical manifestations of pruritus, whealing, flushing, diarrhea, abdominal pain and disorders of cognitive function. By contrast, in all 19 trials with placebo, there was no improvement in these symptoms and signs. Histaminuria and peripheral-blood eosinophilia were unrelated to disease activity and were unaffected by drug therapy. Although it is poorly absorbed after administration by mouth, disodium cromoglycate is of clinical benefit to patients with systemic mastocytosis.