The most frequent cause for an abnormal result during screening of newborn infants for galactosemia is double heterozygosity for Duarte variant and galactosemia, in which galactose-1-phosphate uridyl transferase activity is reduced to approximately 17% of normal. Thirty-nine oral galactose tolerance tests were performed in 27 infants and children with this condition. In comparison to age-matched controls, all children with this genetic variant reached much higher levels of blood galactose and galactose-1-phosphate following oral galactose challenge. The integrated plasma galactose response increased with the age of the child, whereas integrated erythrocyte galactose-1-phosphate responses were elevated to the same degree at all ages. Although all children appeared clinically normal, the marked abnormalities in the ability to dispose of ingested galactose raise questions concerning appropriate dietary recommendations for such children.