A relative intolerance of lactose was demonstrated in 14 of 16 patients receiving nasogastric tube feedings following surgery for head or neck carcinoma. Two liquid diets differing only in the presence or absence of lactose, were administered for eight to 20 days per patient in a double-blind crossover study. Increased stool frequency, decreased stool consistency, and more frequent flatulence, borborygmi, and distention were found with the lactose diet, although classic lactose intolerance (plasma glucose rise of less than 26 mg/dl) was indicated in only two patients. Differences between amounts and times of peak rise following the glucose tolerance test and lactose tolerance test for each patient indicated the advisability of locating patients on a continuum of ability to hydrolyze lactose. The results indicate that lactose should be reduced or eliminated from tube-feeding diets to improve patient tolerance and comfort and to reduce diarrhea.