The growth of 133 children participating in the Collaborative Study of Children Treated for Phenylketonuria (PKU) was compared to growth data from the National Center for Health Statistics (NCHS) to determine whether the growth patterns of the children with PKU were the same as those of unaffected children. Height and weight by age, and weight by height, were analysed for ages 2-10 years. Head circumference by age was analysed for ages 2-7 years. Median height by age of the PKU children was consistently near the 50th percentile of the NCHS growth curves for males and females. However, for both sexes, median weight by height and by age was between the 50th and 75th percentiles for children over 3 years old. Two-sample t-tests showed mean weight was significantly different (p less than 0.05) between the PKU and NCHS groups at most ages for both sexes. Median head circumference for the PKU children tended to be smaller than NCHS standards; however, the maximum difference at any age was less than 0.5 cm. Polynomial growth curves fitted to the PKU and NCHS growth data showed that, on average, the PKU males and females weighed more than their unaffected counterparts, while height and head circumference for both groups were very close. Including median serum phenylalanine (phe) level (mg/dl) in the growth curves suggested that the weight differences between the PKU and NCHS groups are related to degree of diet adherence. Higher phe levels in the PKU group were associated with higher weight levels, more so for girls (p less than 0.001) than for boys (p = 0.08). No relationship was found between phe level and height or head circumference. We conclude that growth in children treated for PKU differs from national standards for weight by age and weight by height, but not for height by age. We speculate that diet adherence may be an important factor in determining which children have a tendency to become overweight.