W e have studied the size and orientation of mineral crystals in cortical bone of oim/oim mice, which are known to produce only alpha 1(I) collagen homotrimers and which may serve as a model for human osteogenesis imperfecta. Long bones (femur and tibia) from young (5 wk old) oim/oim mice and from unaffected heterozygous counterparts were investigated by small-angle x-ray scattering (SAXS), which is sensitive to structures smaller than 50 nm. Mineral crystals were compared in terms of their thickness and their alignment with respect to the long bone axis. While electron microscopic tomography has recently shown the existence of large mineral blocks (with all dimensions typically exceeding 50 nm) in mineralized tendons of oim/oim mice, SAXS revealed a family of thin, possibly needle-like, crystals in cortical bone. These crystals were similar in shape to those observed previously in normal mice, but they were thinner and less well aligned in oim/oim mice relative to heterozygotes. Moreover, the crystal thickness and their alignment with the bone axis were more variable in oim/oim bone, with a close correlation (r = 0.94, P < 0.001) between the two parameters. The presence of smaller crystals with more variable alignment in corticalis of oim/oim mice may contribute to the brittleness of their bone, similar to that of human osteogenesis imperfecta.