A preliminary study showed that placental histology was abnormal for babies who subsequently had rickets. The findings--low plasma phosphate concentration; maximum percentage tubular reabsorption of phosphate; and high urinary calcium loss--are those of renal conservation of phosphate in the presence of phosphate deficiency. In a controlled trial no baby receiving phosphate supplements (50 mg per day) had radiological evidence of rickets whereas bone changes were apparent in 42% of the control group. Prenatal deficiency of phosphate, due to placental insufficiency, can be corrected by phosphate supplementation thereby preventing rickets of prematurity.