A female stumptail macaque had signs of intermittant diarrhea and chronic weight loss over a 2-year period during which she was fed a commercial laboratory maintenance diet. Intolerance of this individual to lactose, a ubiquitous constituent of most commercial primate diets, was diagnosed on the basis of lactose tolerance tests and favorable clinical response to dietary carbohydrate substitution with glucose. Oral administration of varying doses of glucose to this animal consistently produced elevations in plasma glucose concentrations exceeding 20 mg/dl, while orally-administered doses of lactose did not cause significant elevations in blood glucose concentrations.