Osteogenesis imperfecta (OI) is an inherited disorder of connective tissue metabolism characterized by fragility of the bones, resulting in multiple fractures. In this study electron microscopic examination of the osteoid region of iliac crest bone biopsy specimens from a 55-year-old man and his three sons (18, 21, and 25 years of age) affected with the common Type I (autosomal dominant) form of OI revealed collagen fibrils consistently smaller in diameter than those from normal age-matched control subjects, with the majority of OI fibrils measuring 0.04-0.06 micron, as opposed to 0.06-0.08 micron in the control subjects. This defect may be due to the biosynthesis of Type III collagen, not normally found in adult lamellar bone or, more likely, to a failure of maturation of Type I fibrils to their normal diameters, possibly owing to the increased levels of hydroxylysine previously reported in OI collagen, a factor known to be inversely related to fibril diameter.