PIP: The role of Ascaris lumbricoides infection in human nutrition was investigated in 12 children 5-10 years of age. Daily levels of dietary protein intake varied from 1-2.8 g/kg body weight but were kept constant for each child as was caloric intake. Nitrogen, fat balance, and d-xylose absorption determinations were done before and after deworming. A mean reduction in fecal nitrogen excretion of 230 mg/day was noted after worm removal. In 7 children harboring 48 or more parasites, the reduction in fecal nitrogen amounted to 7.2% of the dietary nitrogen. In 3 children receiving 1 g of protein/kg body weight, the improved nitrogen absorption after deworming led to an increased nitrogen retention. In 8 children the infection was associated with moderate steatorrhea (13.4% of dietary fat) and impairment of d-xylose absorption; the former was markedly reduced, and the latter was partially improved following treatment. It is concluded that Ascaris infection in children can lead to marked nutritional impairment when a high parasite load is associated with a low protein intake.