The influence of malabsorption of lactose, as a result of primary lactase deficiency, on the absorption of the nutrients in milk was tested in four healthy controls and four subjects with lactase deficiency. An ileal perfusion technique was used to quantify arrival in the ileum of nutrients and a nonabsorbable marker (polyethylene glycol, PEG 4000) ingested as a test meal of milk. The meal was 250 ml of whole milk or milk in which the lactose had been hydrolyzed to glucose and galactose. In the fasting state, ileal flow of volume, protein, carbohydrate, and electrolytes was small and not different in controls and lactase-deficient subjects. Ileal flow increased in all subjects after the test meal of milk; more fluid and nutrient was recovered from the ileum in lactase-deficient subjects after whole milk than in control subjects or in lactase deficiency after hydrolyzed milk. Two deficient subjects showed marked malabsorption of lactose (35 and 50%); two did not. Protein, calcium, magnesium, and phosphorus were also recovered from the ileum in greater quantities in lactase deficients after whole milk. However, apart from decreased absorption of lactose, the nutritional consequences of malabsorption in association with primary lactase deficiency in adults are probably minimal.