The pharmacokinetics of 186Re-HEDP, a radiopharmaceutical for palliative treatment of metastatic bone pain, was investigated in 11 patients (17 studies) who suffered from metastatic breast or prostate cancer. Half-life times of 186Re in three blood fractions (whole blood, plasma and plasma water) were 40.1 +/- 5.0, 41.0 +/- 6.0 and 29.5 +/- 6.4 hr, respectively. Time-dependent increase in plasma-protein binding was observed, probably caused by in vivo decomposition of 186Re-HEDP. Total urinary 186Re excretion was 69% +/- 15%, of which 71% +/- 6% was excreted in the first 24 hr after injection. The BSI (i.e., fraction of the skeleton showing scintigraphic evidence of metastatic disease) closely correlated with the fraction of dose non-renally cleared (r = 0.98). This implies that the amount of radioactivity taken up by the skeleton and hence the bone marrow absorbed dose can be predicted from a diagnostic pre-therapy 99mTc-HDP scintigram. The pharmacokinetic behavior indicates that 186Re-HEDP has suitable properties to justify its application.