Bone mineral content (BMC, g/cm) and bone mineral density (BMD, g/cm2) of the proximal tibia were determined by dual photon absorptiometry (DPA). Measurements just distal to the subchondral plates of the tibia condyles, where the bone structure is predominantly trabecular, proved to give the most consistent results. The precision of BMC measurements in this region, expressed as the coefficient of variation, was 1.1 per cent and of BMD measurements 2.5 per cent. In a cross-sectional study on 63 normal women and men, BMC and BMD showed a decrease with age at a rate of about 8 and 9 per cent per decade, respectively, in women, but not in men. In normal women, BMC of proximal tibia was correlated with BMC of lumbar spine, femoral neck, and femoral shaft, as well as with body weight and height. DPA may be useful in the study of bone reactions, such as in patients undergoing arthroplasty of the knee.