Bone uptake of 99Tcm-MDP was evaluated 4 h after injection in 27 normal subjects aged 23-50 years by gamma camera measurement of the activity in whole skeleton regions of interest (ROIs) in anterior and posterior projection. The mean global skeletal uptake (GSU) (% of whole-body activity in both projections measured 30 s after injection) was 33.5 +/- 4%. The standard urinary excretion method (whole-body retention, WBR) gave a mean bone uptake value of 31.5 +/- 4%. A significant correlation (r = 0.570; P less than 0.002) was found between GSU and WBR. The mean bone-to-soft tissue ratio (B/S index) was 3.0 +/- 1.1. It is suggested that this direct, external counting method provides a way of obtaining both a qualitative and a semiquantitative evaluation of bone uptake with a single tracer administration in a routine bone scan. Investigation of its use in the assessment of bone involvement in low and high turnover bone diseases is now under way.