Disturbed mineral and bone metabolism has been reported to occur frequently in very-low-birth-weight infants fed breast-milk during the first 3 months of life. This study was designed to assess the prevalence of disturbed mineral homeostasis in a breast-milk-fed very-low-birth-weight population at Baragwanath Hospital and to determine whether the addition of a preterm infant formula to the feeds would reduce this prevalence and increase the rate of weight gain. Fifty-three neonates weighing less than 1 200 g at birth were monitored for weight gain, growth and biochemical and radiological evidence of metabolic bone disease at 2-weekly intervals during hospitalisation and for 18 weeks after discharge. The infants were randomised at 2 weeks of age to receive either breast-milk only, or a combination of breast-milk and a premature formula containing 550 mg calcium and 300 mg phosphorus. All infants received 800 IU vitamin D daily from day 14. Weight gain and growth were similar in both groups. Calcium and phosphorus intakes were higher in the mixed feeding group, but did not affect serum mineral levels. Radiological rickets was uncommon in both groups although periosteal reactions and osteopenia occurred frequently and with similar prevalences. Vitamin D deficiency was not found to be a problem. In conclusion, overt rickets is not a major problem in very-low-birth-weight infants born at Baragwanath Hospital, although biochemical abnormalities occur frequently.(ABSTRACT TRUNCATED AT 250 WORDS)