The assumption that children with phenylketonuria (PKU) develop normally until birth was brought into question by the recent report that PKU children weigh several hundred grams less at birth than their unaffected siblings. We have examined intrafamily differences in birthweight in 40 sibships with at least one affected and one unaffected child. The difference in mean birthweights computed by taking a weighted average of the intrafamily differences was –69 gm, and the adjusted estimate of the birthweight difference between children with PKU and their siblings obtained from a fitted multiple regression function, is –51 gm. The findings are not consistent with the large difference in birthweight reported previously and are compatible with the assumption that the intrauterine physical growth of children with classical PKU is not adversely affected.