Fifty-five patients with bone lesions underwent 201Tl-chloride (201TlCl) scintigraphy to evaluate its findings and usefulness for the diagnosis of bone metastases. 201TlCl scintigraphy was performed 15 minutes (early scan) and 2 hours (delayed scan) after intravenous administration of 5.55 MBq/kg of 201Tl-chloride. To evaluate the degree of tracer retention at the lesions, we calculated retention index after setting regions of interest in each lesions demonstrated tracer uptake in both early and delayed scans. Among 118 lesions of pathologically and/or clinically confirmed bone metastases, 201TlCl planar images disclosed 90 (76.3%) and 88% (74.6%) lesions on early and delayed scans, respectively. SPECT images were preferable for the evaluation of the lesions in the spine, the thoracic cage and pelvis. Correlation of tumor size with findings on 201TlCl images demonstrated 90.5% sensitivity for the metastases more than 7 cm3. No correlational difference was seen in the sensitivity depending on primary cancers. The overall retention index of bone metastases was -21.4 +/- 47.1. Bone metastases of pulmonary adenocarcinomas and small cell carcinomas demonstrated higher retention index than those of pulmonary squamous cell carcinomas and breast cancers. On 201TlCl images, no abnormal tracer uptakes were seen in benign lesions detected by 99mTc-hydroxymethylene diphosphonate (99mTc-HMDP) images except 5 lesions. The retention index of these benign lesions demonstrated abnormal uptakes on 201TlCl images was -48.3 +/- 15.0, which found no significant difference between that of metastases. In cases of vertebral compression fractures, abnormal tracer uptakes were demonstrated in 9 of the 12 pathologic fractures against 2 of the 10 benign ones. Those 2 benign fractures proved to be vertebral tuberculosis.(ABSTRACT TRUNCATED AT 250 WORDS)