A simple method for the determination of cross-section area of diaphyseal cortex is presented. Measurements of the area of rib cortex in specimens from 139 relatively normal patients showed that the minimum area occurred in infancy with a gradual rise toward late adolescence and early adulthood. A sharp decline followed. At age 35, the decline in area became less pronounced. The minimum adult values reached in the eighth decade were approximately 25% less than those of the early twenties. Accompanying measurements of maximum outer diameters showed that these changed little after adult values are attained. The authors concluded that enlargement of the marrow space due to an excess of endosteal resorption in relation to endosteal formation accounts for the decline in area. That the rate of decline changed at age 35 was surprising. It is believed that a fundamental change occurs in skeletal physiology at this time.