Cortical bone mass was quantified in 1278 Nigerian children (695 males and 583 females) aged 3 16 years in a prospectively designed cross-sectional and longitudinal study. The total bone width (T) and medullary cavity width (M) were measured at the midshaft of the second metacarpal bone using a direct reading caliper. From the above measurements the cortical width (C), cortical area (CA), and percent cortical area (PCA) were calculated using the method of Garnet al. [6] and showed a progressive increase of T, C, CA and PCA, reaching a plateau at 15 years. At all ages, the values for both T and M are higher in males than in females. On the other hand, and contrary to established normal values amongst both white and black Americans, between the ages of 9 and 15 the female values for C are higher than those for males. This difference is greatest at the age of 12 years (p <0.001). The implication of this finding may be that during these years, African male children do not compensate for the increased endosteal resorption with a greater total cortical width (T), since the medullary width in females remains relatively constant over the years.