Bone scintigrams made with Tc-99m-tagged HEDP (1-hydroxyethylidene diphosphonate) and MDP (methylene diphosphonate) were compared, the former at 4 h after injection, the latter at both 2 and 4 h. In 17 patients with skeletal metastases, there was no significant difference in lesion count or scan quality between the 4 h images. The tumor-to-bone ratio (T/B) was significantly higher with Tc-HEDP (P < 0.02). Lesion detection rate and T/B ratios were both lower with Tc-MDP at 2 h when compared with the 4 h values for both Tc-HEDP (P < 0.02, P < 0.005) and Tc-MDP (P < 0.02, P < 0.01). The 4 h Tc-MDP scan was of significantly higher quality than the 2 h Tc-MDP scan (P < 0.01). Although Tc-HEDP produces a higher T/B ratio at 4 h, neither agent seems to be superior in clinical practice.