The relationship and possible disproportion between different cranial dimensions--head length, head circumference and biparietal diameter--body length and developmental status in normocephalic children were studied in 166 mentally retarded and 471 normal control subjects, between the ages of 3 months and 6 years. When the total cohort was analyzed, all dimensions, particularly head length, were found to be significantly reduced in the study group compared with the controls. Stratification into three age-groups revealed that in the 3- to 15-month-old subjects, head length was the most significantly reduced dimension, while in the older children body length was more significant, followed by head length. Further discriminant analysis resulted in a formula consisting of only three factors--body length, head length and age. Head circumference and biparietal diameter were noncontributory. This investigation did not support the concept of a disproportionately small head compared with height in normocephalic mentally retarded children, but rather a general growth failure mostly affecting height and head length.