Twice weekly plasma and urine measurements were made in 24 very low birth weight infants. Intravenous feeding was given whilst infants required respiratory support. Subsequently they received breast milk or formula milk with a vitamin D supplement of 400 U/day. Fourteen babies required intravenous feeding for more than 10 days. Six developed radiological rickets or severe osteoporosis, and these infants had plasma phosphate levels < 1 · 2 mmol/L on breast milk or < 1 · 8 mmol/L on formula milk. Babies without radiological rickets had plasma phosphate levels > 1 · 2 mmol/L on breast milk and > 1 · 8 mmol/L in all but one on formula milk. Successful treatment of rickets was associated with a rise in plasma phosphate to the above levels. Untimed urine calcium and phosphate concentrations expressed as creatinine ratios were not helpful in detecting babies with rickets, but may be useful in monitoring therapy.