The value of the bone scan in pyogenic bone and joint infections is demonstrated in patients who had signs and symptoms suggestive of bone or joint infection. Nineteen patients were evaluated with 99m Tc-methylene diphosphonate bone scan and roentgenogram. The diagnosis of acute osteomyelitis was made in eight patients, chronic osteomyelitis in six patients, septic arthritis in three patients and two patients had soft tissue infection only. Seven of the eight patients with acute osteomyelitis had focal increase of radiopharmaceutical uptake in the bone well before the bony change appeared on roentgenogram. Five of six chronic osteomyelitis patients had not only bony change on roentgenogram but also increased radiopharmaceutical uptake of bone. But the remaining one had only the former, and the lesion was interpreted as inactive. In two of three septic arthritis patients, the lesion was in S-I joint and both of them had no abnormality on roentgenogram but had increased uptake of radiopharmaceutical agent in the joint. Two patients with soft tissue infection had no abnormal radiological bony change and no increase of the radiopharmarceutical uptake in bone on bone scan. From the above data, we concluded that bone scan is recommended in the evaluation of the patients with signs and symptoms suggestive of bone or joint infection for the earlier diagnosis and differential diagnosis in acute case and for the determination of the activity and location of the lesion in chronic case.