One hundred sixteen healthy black children ages 13 to 59 months, representing high and low socioeconomic deciles, were studied for lactose malabsorption. A fasting lactose tolerance test using 2 g of lactose/kg of body weight was carried out. Glucose was determined at 0, 15, 30, and 60 min. Of the 116 preschoolers 34 (29%) evidenced lactose malabsorption as determined by a blood glucose rise of less than 26 mg/100 ml. Clinical signs of diarrhea, gas, and cramps were noted singly or in combination in 18% of the 34 lactose-malabsorbing children. Of the 82 lactose absorbers, 12% demonstrated similar signs. The nature and length of the initial infant milk feeding failed to show any relationship to the onset of malabsorption. Current milk drinking patterns were reported as being similar. Eight-seven percent of the malabsorbers and 92% of the absorbers report drinking 240 ml or more of milk/day. Socioeconomic status, education, marital status, and medical assistance of the parent is similarly distributed between lactose absorbers and malabsorbers.