Objective We report the findings of Thallium-201 chloride (²⁰¹TlCl) scintigraphy and consider how to use this technique to evaluate the character of soft tissue lesions. Patients and Methods We enrolled 91 consecutive patients (45 males and 46 females, age range 8-91-years-old). Nineteen patients were malignant and 72 were benign. Patients were scanned 15 minutes (early phase) and 3 hours (delayed phase) after ²⁰¹TlCl injection. More intense uptake in the lesion compared to the normal side was considered as ‘high’, the same degree of uptake was considered ‘iso’, and decreased uptake was ‘low’. The retention index (RI) was calculated in 9 patients in the malignant group and in 16 patients in the benign group. Results In malignant tumors, 15 of 19 patients showed high uptake in both the early and delayed phases. One malignant fibrous histiocytoma patient was high only in the delayed phase and 1 liposarcoma patient was high only in the early phase. Two liposarcoma patients showed an iso uptake in both phases. One of these patients was pathologically diagnosed as a myxoid type. In benign lesions, no lipoma showed increased uptake. All neurogenic tumors except for 2 demonstrated high uptake. All 3 ganglions of the lower extremities showed iso uptake. Most inflammatory diseases showed increased uptake. Clinically-considered benign patients consisted of tumorous lesions or inflammatory disease. Only 2 patients were considered ‘low’, and these were diagnosed as intramuscular hematoma and cyst. RI was variable in both malignant and benign lesions and no statistically significant difference was seen between malignant and benign lesions by t-test (p=0.72). Conclusions A high ²⁰¹TlCl uptake lesion is more frequently seen in malignant tumors, but regardless of whether the tumor is benign or malignant, according to the histopathological variety, the ²⁰¹TlCl uptake pattern can not be the only indicator to differentiate malignant from benign tumors. We ultimately need to evaluate the nature of tumors by a combination of several imaging techniques. J. Med. Invest. 56: 136-141, August, 2009