The prevalence of biochemical abnormalities usually associated with rickets was investigated in three black school-going populations, one from a rural community, one from a small urban community, and one from a large urban area. A high prevalence of biochemical abnormalities was found in the rural community, where 13.2% of children were hypocalcemic and 41.5% had elevated alkaline phosphatase concentrations. No hypocalcemia was detected in the children from the large urban area. Urinary calcium excretion was lowest in the community with the highest prevalence of hypocalcimia and elevated alkaline phosphatase concentrations. Dietary calcium intake in those children with biochemical abnormalities was estimated at 125 mg/day, compared with 337 mg/day in those children with normal biochemistry. It is suggested that the pathogenesis of the biochemical abnormalities in the rural community is due to a low dietary intake of calcium.