Predicting radionuclide bone scan findings in patients with newly diagnosed, untreated prostate cancer: prostate specific antigen is superior to all other clinical parameters. Presently, the standard staging evaluation of prostate cancer includes digital rectal examination, measurement of serum tumor markers and a radionuclide bone scan. To evaluate the ability of local clinical stage, tumor grade, serum acid phosphatase, serum prostatic acid phosphatase (PAP) and serum prostate specific antigen (PSA) to predict bone scan findings, a retrospective review of 521 randomly chosen patients (mean age 70 years, range 44 to 92 years) with newly diagnosed, untreated prostate cancer was performed. Local clinical stage, tumor grade, acid phosphatase, PAP and PSA all correlated positively with bone scan findings (p less than 0.0001). Using receiver operating characteristic curves, however, PSA had the best over-all correlation with bone scan results. The median serum PSA concentration in patients with a positive bone scan was 158.0 ng./ml., whereas men with a negative bone scan had a median serum PSA level of 11.3 ng./ml. (p less than 0.0001). Using multivariate logistic regression analysis, local clinical stage, tumor grade, acid phosphatase and PAP were evaluated in combination with PSA to assess whether these parameters increased the ability of PSA alone to predict bone scan findings. None of these clinical parameters, irrespective of the combination used, contributed appreciably to the predictive power of PSA alone. A probability plot with 95% confidence intervals was constructed that allows the practicing urologist to estimate on an individual basis the probability of a positive bone scan for any given serum PSA value. The most significant finding of this study, however, was the negative predictive value of a low serum PSA concentration for bone scan findings. In 306 men with a serum PSA level of 20 ng./ml. or less only 1 (PSA 18.2 ng./ml.) had a positive bone scan (negative predictive value 99.7%). This finding would suggest that a staging radionuclide bone scan in a previously untreated prostate cancer patient with a low serum PSA concentration may not be necessary. 