Even on a lactose-restricted diet, urinary galactitol excretion and erythrocyte galactose-1-phosphate levels are persistently elevated in patients with galactose-1-phosphate uridyltransferase deficiency. In order to determine the contribution of galactose in dietary fruits and vegetables to this phenomenon, (1) the content of galactose in a lactose-free diet was directly measured when a galactosaemic patient's diet was specifically enriched in those fruits and vegetables which contain relatively large amounts of free galactose and (2) galactitol excretion was determined during ingestion of this diet for 3 weeks and while on a synthetic diet for 1 week that provided < 8 mg galactose/day. For comparison the effect of a 3-week supplementation of 200 mg galactose/day was determined. The measured intake in total foodstuffs matched the theoretical content of galactose in the patient's diet based on amounts in fruits and vegetables alone, thus supporting the concept that fruits and vegetables are primarily responsible for galactose intake in a lactose-free diet. All of the dietary manipulations, however, had relatively little effect on metabolite levels, suggesting that endogenous galactose production is primarily responsible for the elevated levels of galactose metabolites routinely detected in patients on lactose-restricted diets.