Two studies were conducted to investigate the contribution of Ascaris lumbricoides to lactose maldigestion in preschool children in two different communities in Panama where milk is available as a source of nutrients and the prevalence of Ascaris is known to be high. Both Ascaris-infected and uninfected children were given a standard lactose load and lactose absorption was studied by measuring the rise in plasma glucose in study 1 and by determination of breath hydrogen concentrations at regular intervals after ingestion of the test dose in study 2. All children were tested before anthelmintic treatment with levamisole and 3 wk after therapy. The mean rise in blood glucose from infected (n = 13) children 40 min after the ingestion of lactose was about half of that of the controls (n = 21). After deworming, lactose digestion improved in previously infected children. In study 2, significant differences in breath hydrogen concentrations postmilk ingestion were observed between the Ascaris-infected (n = 47) and the uninfected children (n = 35) before treatment. There was a substantial reduction of breath hydrogen after milk ingestion in the previously infected children after therapy. No differences were observed in breath hydrogen content of the uninfected children during the pre- and posttreatment phases of the study in the lactose tolerance test. These studies provide evidence that infection with Ascaris lumbricoides impairs lactose digestion in preschool children.