A case of uridine diphosphate galactose (UDP-Gal) 4-epimerase deficiency was discovered by mass screening of newborn infants. UDP-Gal 4-epimerase activity of red blood cells from the patient was found to be remarkably low, i.e., 7.5% of the level in normal controls at comparable ages. The parents showed intermediate values between those of the patient and controls. The enzyme activity in a specimen of liver tissue obtained from the patient by needle biopsy revealed a normal value. Subsequently, two other families with the condition were found by mass screening and these individuals were found to be heterozygotes.