AbstractThis study was conducted on X‐ray films of the shaft of normal femur and second metacarpal bones of adult male and female Nigerians to compare the percentage cortical thickness relative to the subperiosteal diameter (cortical index) of these bones. Age was correlated with cortical index (CI) using Pearson's product of moment correlation coefficient. Confidence limits of CI were calculated for clinical use. Results obtained were analysed for each sex and compared atP<0.05 level of significane. CI of the second metacarpal is significantly greater than of the femur in both the male and female. Age maintained significant inverse correlation with CI of the femur and nonsignificant correlation with the second metacarpal in the female. In the male, age had nonsignificant correlation with the femur or with the second metacarpal bone.It is concluded that (i) the amount of percentage relative cortical bone to subperiosteal diameter of the second metacarpal, a nonweight‐bearing bone, is greater than that of the femur, a weight‐bearing bone, (ii) age of the adult has significant effect on the amount of cortical bone of the femur in the female but not in the male, and (iii) age of the adult does not have significant effect on the amount of cortical bone of the second metacarpal in both