A simple technique for determining the whole body retention of 99mTc labeled methylene diphosphonate (MDP) at 4-h and 24-h was applied to 28 adult patients (22 prostate cancer, 6 osteoporosis). Following administration of 20 mCi (740 MBq) 99mTc-MDP, a qualitative scintigram was performed and whole body retention was recorded at 4 and 24 h. The prostate cancer patients all had positive bone scintigrams, and of this group, 7 were in relapse and 15 in remission on chemotherapy. The osteoporotic groups whole body retention values were determined prior to the initiation of drug therapy. Mean percent whole body retention values were significantly greater at 4 and 24 h for the cancer patients in relapse relative to both the prostate cancer patients in remission and the osteoporotics: 4-h%/24-h%; equals 74.3/60.2, 57.5/33.5, and 48.0/30.3, respectively. The whole body retention values of the latter two groups, however, were only significantly different at 4-h. An additional index of skeletal pathology was developed by combining the 4-h/24-h values to calculate the biological half time (t1/2b) for this time period using a single exponential model. Again the cancer patients in relapse exhibited a significantly longer half time relative to those in remission and the osteoporotics (70.4 vs 25.7 h and 29.2 h, respectively). The combination of 4 and 24-h whole body retention values with the associated t1/2b has potential for following and classifying patients during the course of their skeletal disease. These values have been shown to be especially valuable with cancer patients with metastatic disease.