Nineteen of 20 healthy Oriental adults living in the United States developed abdominal cramps and diarrhea after ingesting an amount of lactose equivalent to that in one quart of milk; 14 reported similar symptoms after one or two glasses of milk; all had consumed milk as infants without having such symptoms. Two of 20 Caucasians tested were intolerant to milk and lactose. Many Orientals therefore may have a genetically determined lactase deficiency that may lead to intolerance to milk. Since lactase deficiency is also common among Negroes, the bulk of the world's adult population is probably intolerant to milk.