An investigation was carried out on 61 children suffering from symptomatic giardiasis with the object of verifying the incidence and entity of lactose malabsorption. Furthermore, the possibility of a substitutive yogurt diet was verified in the lactose malabsorbers. The subjects, all children older than 1 year, were studied according to a schedule that included a lactose hydrogen breath test (BT) performed prior to therapy and a further BT 60 days following therapy. The subjects were divided in two groups: group A, 40 children, received a dose of 250 ml of cow's milk; group B, 21 children, received a stress dose of 2 g/kg lactose (max 50 g). Those subjects who were lactose malabsorbers at the 60 day follow-up were also given a BT at 75 days, and in the case of persistent malabsorption, a further BT was performed after 24 h with the administration of yogurt (450 g containing 12.1 g of lactose). Furthermore, 40 subjects matched for age and sex but without any GI complaints served as controls. The results showed lactose malabsorption to be frequent in children with Giardia lamblia symptomatic infection. According to the BT with a standard lactose load, all patients were malabsorbers; when testing lactose absorption with 250 ml of cow's milk, 45% of patients were found to be malabsorbers. In the latter subjects, the oral load of yogurt was uniformly well tolerated and gave rise to no H2 increment on the BT. We conclude that the occurrence of lactose malabsorption of nutritional relevance is common in children suffering or having suffered from giardiasis.(ABSTRACT TRUNCATED AT 250 WORDS)