At discharge from hospital, 36 preterm infants (birthweight < 1,800 g) were randomized to be given either a standard formula or an identical formula enriched with calcium (47 vs. 80 mg/100 ml) and phosphorus (30 vs. 45 mg/100 ml) exclusively until they reached a corrected age of 3 months, thereafter beikost and other formulas were introduced into the diet. Weight, head circumference and radial and body length were measured and radial bone mineral content (BMC) and bone width (BW) determined by single photon absorptiometry at discharge, at 3 and at 6 months of corrected age. Additionally, an X-ray of the wrist was taken and serum alkaline phosphatase activity determined at discharge, and the latter repeated at 3 months. Bone mineral content, bone width, and the individual increments of both parameters were significantly higher in infants fed the enriched formula at 3 months of age, but these differences were less pronounced at 6 months.