Lactose digestion and tolerance were evaluated in 164 African Americans ranging in age from 12 to 40 y who claimed intolerance to one cup (240 mL) or less of milk. With use of a breath-hydrogen test with 25 g lactose as test dose and the presence or absence of symptoms, 50% of the subjects were classified as lactose maldigesters and intolerant, 8% were maldigesters but tolerant, 15% were digesters but intolerant, and 27% were digesters and tolerant. Forty-five subjects from the lactose maldigesting and intolerant group were further tested for milk intolerance in a double-blind study. Sixty-seven percent of the subjects reacted appropriately to the presence or absence of lactose in ingested milk whereas 33% reported symptoms to both low-lactose milk and milk containing lactose. The results suggest that the cause of milk intolerance in as many as one-third of African Americans claiming symptoms after ingestion of a moderate amount of milk cannot be its lactose content.