The delayed images of the 4-phase 99mTc phosphonate bone scan are compared with the delayed images of the 3-phase study in patients with diabetes mellitus and/or peripheral vascular disease and suspected osteomyelitis. Three-phase bone imaging includes an immediate postinjection radionuclide angiogram, a blood-pool image and delayed static images to 7 h. The 4-phase study adds a 24-h static image. The scan is positive for osteomyelitis if images show progressively increasing lesion to background activity ratios over time. The results of analyzing 21 three- and four-phase bone scans in 17 patients were correlated with clinical course, cultures and/or X-rays, Ga scans and CT [computed tomography] scans. The accuracy of 4-phase bone imaging for diagnosing osteomyelitis was 85%; for 3-phase; 80%. Sensitivity for 4 phase was 80%; specificity was 87%. Sensitivity for 3 phase was 100%; specificity was 73%. Since overall accuracy of the 4-phase study is slightly better than 3-phase, in these patients with diabetes mellitus and/or peripheral vascular disease, the addition of a 24-h image, creating a 4-phase bone scan, is recommended.