The role of lactose malabsorption was studied prospectively in 80 schoolchildren with recurrent abdominal pain. Malabsorption was documented in 40 per cent (16 of 59 whites, 12 of 16 blacks and four of five Hispanic children) on the basis of elevated levels of hydrogen in their breath. Those with lactose malabsorption, however, were not clinically distinguishable on the basis of past milk ingestion (P greater than 0.05), weekly pain frequency (median, five vs. six times), presence of diarrhea (40 vs. 27 per cent) or symptom response to lactose load. In children with malabsorption who completed a six-week diet trial, 70 per cent reported increased frequency of pain (P less than 0.002) when placed on their usual lactose-containing diet. Lactose malabsorption has a substantial role in the symptoms of children with recurrent abdominal pain, and it should be considered before performing invasive procedures or assuming a psychogenic origin.