Head circumference graphs for boys and girls from birth to 18 years of age, calculated from reports in the world literature published since 1948, are presented. No significant racial, national, or geographic differences in head circumferences were found. A single "normal" head circumference, that is one which lies within two standard deviations above and below the mean for age and sex, may prove misleading, since it gives no clue as to rate of head growth. Serial head circumference measurements should, therefore, be a routine part of the physical examination of infants and children. When such measurements are plotted on the composite graphs, abnormal growth patterns are readily discernible. Rapid upward deviation may signify correctable conditions, such as hydrocephalus, subdural hematomas or effusions. Marked slowing or arrest of head growth offers a poor prognosis for mental development. But even a single measurement outside the range of normal should lead to further evaluation of the child.