The interactive effects of severe diabetes and insulin therapy on the geometrical, biomechanical, and histomorphological characteristics of the femoral neck were studied in rats that had streptozotocin-induced, insulin-dependent (Type-I) diabetes. Thirty-six female Sprague-Dawley rats (8 weeks of age) were assigned randomly to one of three groups: 12 to control (C), 12 to severe diabetes mellitus (SDM), and 12 to severe diabetes with insulin treatment (SDI). At the conclusion of 10 weeks, the femoral necks were loaded to failure via cantilever-bending tests, and the geometrical, structural, and material properties of the femoral neck were measured and correlated with fracture-surface cross sections. Decalcified cross sections of the femoral necks were analyzed histomorphometrically to determine the porosity, the bone-cell counts, and the bone spicule/marrow space ratio. Rats with severe insulin-dependent diabetes had significantly lower total body mass than did control rats, as well as significantly less femur mass, femur length, total-bone cross-sectional area, and cortical-shell cross-sectional area. Insulin therapy ameliorated some, but not all, of the detrimental effects of diabetes on femoral neck geometry. Compared with control and SDI rats, SDM rats had lower values for femoral neck structural properties, although differences in structural properties may have been related to retarded growth as well as to diabetes. SDM rats had a significantly lower bone spicule/marrow space ratio and number of osteoclasts than did either the control or SDI rats and had significantly greater porosity in the femoral neck cortex than did control rats. Decrements in femoral neck material properties--which were independent of differences in body size--were significantly and linearly correlated with severity of diabetes (as measured by blood glucose level).