The bone mineral content of 109 lumbar vertebrae was related to the number of trabecular microcalluses in the vertebral bodies. The appearance and location of the calluses were also investigated. Microcalluses were found in 87 of the vertebral bodies. The incidences of calluses varied in the different groups (31-79 years) between 60 and 90%. The number of calluses increased with decreasing bone mineral content. The location of the calluses corresponded to the location of experimental in vitro compression fractures of the vertebral bodies.