A 16-year-old boy with systemic mastocytosis from birth had a lifelong history of pronounced diarrhea. Treatment with ingested disodium cromoglycate (DSCG) restored a normal bowel movement pattern and this has been maintained for the six months he has been on treatment. Changes in dosage have repeatedly demonstrated a close relationship between the dose of ingested DSCG and the control of the diarrhea. This case report indicates the potential efficacy of ingested DSCG in the treatment of some gastrointestinal disorders.