Rickets was diagnosed clinically and confirmed radiologically in a preterm infant at 12 weeks of age, who despite therapy with vitamin D died. Four preterm infants who were subsequently admitted to the nursery had their biochemistry followed serially as controls. All infants demonstrated a marked increase and then subsequent decrease in plasma alkaline phosphatase activity; the peak value occurring near to what should have been gestationally term. The plasma alkaline phosphatase was of bony origin. Rickets was confirmed radiologically in only one of these four infants. Rising and markedly elevated plasma alkaline phosphatase activity appears to be common in this group of infants and may represent a physiological increase in osteoblastic activity and increasing bone mineralisation in what would have been the infants' 3rd trimester of pregnancy and immediate newborn period.