A study is reported of bone rarefaction in 65 men and 160 women selected at random from old people living at home. They were grouped according to bone thickness (derived from measurements of the second metacarpal cortex made directly from radiographs), and serum alkaline phosphatase. No relationship was found between bone thickness or serum alkaline phosphatase and the following: serum calcium, serum phosphate, intake of calcium, protein, or ascorbic acid, or urinary calcium excretion. Low intakes of vitamin D were frequent, and these were related to bone rarefaction in men and serum alkaline phosphatase in women. Evidence of vitamin D deficiency was thus demonstrated, but its relationship to bone rarefaction is uncertain. No evidence of a separate population with ‘senile osteoporosis’ was found.