Chemotherapy is a recognized cause of morphological alterations to the proximal intestine. Lactose malabsorption, the functional consequence of a small intestinal enzymatic derangement, has been shown to play an important role in causing gastrointestinal symptoms in subjects receiving chemotherapy. To establish a rational basis for the exclusion of lactose from the diet and to reduce the risk of developing gastrointestinal symptoms, we conducted a study of lactose absorption in 20 children during cancer chemotherapy. Because lactose is an important nutritional sugar, the tolerance of lactose provided by yogurt was examined. Lactose absorption was investigated by a hydrogen breath test (BT) after oral ingestion of milk (250 ml) containing physiological doses of lactose (12 g). The effect of yogurt supplementation was also tested by BT after meals of yogurt (450 g) also containing physiological doses of lactose (12.1 g). In 11 children, lactose malabsorption was detected by BT during the study before any gastrointestinal symptom revealed this status. Of these 11 children, no gastrointestinal discomfort developed in five receiving a lactose-excluded diet. In contrast, in the six children not restricted in lactose intake, gastrointestinal symptoms were observed 4 to 13 weeks after lactose malabsorption was detected by BT. The findings of our study suggested the usefulness of dietary supplementation with yogurt, a lactose-containing food, in children who developed lactose malabsorption. In fact, all lactose-malabsorbent children showed good lactose absorption and tolerance when tested by yogurt BT. Furthermore, a yogurt-supplemented diet was also successful in three subjects initially recognized as lactose intolerant. In conclusion, the data presented have important implications in reducing the risk of developing gastrointestinal symptoms in children receiving cancer chemotherapy.