Bone mineral content of the forearm was measured by photon absorptiometry in 17 preterm infants at a postconceptional age of 40 weeks. Radiographs of the forearm were assessed by Koo's method and plasma alkaline phosphatase activity was also measured at this time. Bone mineral content was significantly but weakly correlated with Koo score and was not significantly correlated with alkaline phosphatase activity. Neither of these two commonly used investigations accurately predicts the presence of underlying bone disease. Compared with 15 full term infants the preterm infants had significantly lower values of bone mineral content, palpated ulnar length, and crown-heel length. After adjusting for weight and ulnar length the preterm group still had a significantly lower mean value of bone mineral content than the full term group. Accurate diagnosis of osteopenia of prematurity requires photon absorptiometry, with bone mineral content assessed relative to body weight or ulnar length.