Several bone matrix protein constituents, including the major component collagen type I and the hydroxyapatite binding protein, osteocalcin, have been implicated in the regulation of bone turnover. Cortical bone osteocalcin, collagen and mineral content were studied in autosomal dominant osteopetrosis type I (ADO), a disorder characterized by diffuse symmetrical osteosclerosis. Iliac crest bone biopsies were obtained from eight patients (mean age 43.0 years, range 17-63 years) and compared with 16 age- and sex-matched normal controls (mean age 44.1 years, range 20-61 years). The osteocalcin level in cortical bone was increased (p < 0.03) in ADO (51.4 +/- 3.9 mg/kg bone) compared with controls (38.0 +/- 3.6 mg/kg bone). Total collagen, protein and calcium expressed per kilogram bone dry weight were without significant difference between patients and controls. The pathogenesis of ADO is most likely not related to cortical bone osteocalcin content, a protein implicated in osteoclast ontogeny and activation. These observations are in contrast to recent observations of reduced bone osteocalcin levels in osteopetrotic mutations in the rat and underscore the interspecies heterogeneity of this disorder.