Radial diaphyseal bone mineral density (BMD) was measured at the standard one-third site by dual-energy X-ray absorptiometry (DEXA) and by 125I single photon absorptiometry (SPA) in 70 consecutive subjects, aged 12-86 years, with metabolic disorders of the skeleton. Each patient was measured once by the DEXA (Hologic QDR-1000) instrument and four times by the SPA (Norland 2780) instrument on the same day by one or the other of 2 technicians. The DEXA and SPA measurements were linearly related and highly correlated (r = 0.975, P less than 0.0001) over a range from severe osteopenia to high normal BMD. Ninety-five percent of the variation in the BMD determined by SPA was accounted for by DEXA, so that the BMD(SPA) = 1.035 +/- 0.027 (SEM) x BMD(DEXA)-0.007 +/- 0.019 (SEM). This permits continued use of previously accumulated SPA data-bases. The coefficient of variation for repeat measurements by DEXA was 1.2% and by SPA 1.6%. Examination time by DEXA was 6-7 minutes, about 45% shorter than the corresponding SPA determinations. DEXA is the superior method for evaluation of the radius, as it provides faster and more precise measurements in clinical practice.