The skeletal changes in 19 very low birthweight infants (less than 1500 g) were observed from birth to 10 weeks, by means of clinical, biochemical, and radiological techniques. All infants were receiving a supplement of 800 IU vitamin D a day from age 2 weeks. None of the infants showed any specific physical sign of rickets during the period of study. Six infants showed radiological evidence of skeletal demineralisation; 1 of these had severe changes of rickets and 1 had both rickets and fractures. These 6 infants were of shorter gestational periods and lower birthweights than the infants not showing radiological changes. They tended to have more clinical problems and to reach a predetermined volume of feeds (160 ml/kg a day) later than the unaffected infants. Serum alkaline phosphatase values were significantly higher at 5 weeks in the infants with abnormal radiographs than in those without. There were no significant differences between the two groups in relation to serum calcium, inorganic phosphate, 25 hydroxyvitamin D, and immunoreactive parathyroid hormone. The pathogenesis of the skeletal lesions of very low birthweight infants remains unknown.