The relationship between growth and plasma alkaline phosphatase activity differs in the preterm infant when compared with older children and adolescents. 18 preterm infants were studied over the first 12 postnatal weeks; growth velocity and plasma alkaline phosphatase activity were measured serially. Plasma calcium, inorganic phosphate, the vitamin D metabolites and parathyroid hormone were assayed at a median age of 3 and 6 weeks. There was an overall significant negative correlation between growth and plasma alkaline phosphatase activity (r = -0.37; P less than 0.005); this was more marked in a group where the plasma alkaline phosphatase activity exceeded the preterm reference range. Plasma 1,25-dihydroxycholecalciferol and inorganic phosphate were lower in this group at 3 weeks but not at 6 weeks when compared with infants where plasma alkaline phosphatase activity did not exceed the reference range. There was no difference in caloric intake between the groups. In the preterm infant biochemical rickets is common and unlike older children and adolescents an increase in plasma alkaline phosphatase activity correlates with decreased rather than increased growth.