In a study on 857 infants born preterm, high peak plasma alkaline phosphatase activity was independently related to slower growth rate in the neonatal period, and to a highly significant reduction in attained length at 9 months and 18 months post term. At 18 months the deficit in body length associated with peak neonatal plasma alkaline phosphase activity of 1200 IU/l or more was 1.6 cm (95% confidence interval 0.9 to 2.3 cm) after adjusting for confounding factors. The strength and magnitude of this association between high plasma alkaline phosphase activity and body length was greater than that for any other factor identified, including the infant's sex and the presence of fetal growth retardation. Data are presented that support the view that the high plasma alkaline phosphatase activity reflected early bone mineral substrate deficiency resulting in metabolic bone disease. We speculate that even silent early bone disease may interfere with the control of subsequent linear growth and emphasise the potential importance of providing preterm infants, especially those fed human milk, with adequate substrate for bone mineralisation.