The usefulness of 201TlCl SPECT (Tl SPECT) for the diagnosis of breast cancer was evaluated in 14 patients with various breast tumors (9 with invasive ductal carcinoma, 2 with fibroadenoma and 3 with benign process). These tumors ranged in size from 1.5 cm x 1.5 cm to 15.0 cm x 14.0 cm. Tl SPECT was carried out 2 hours after the intravenous injection of 201TlCl (185 MBq). For quantitative study, ROIs were set in the tumor (T), normal tissue of the opposite breast (B) and myocardium (M). Count ratios of T/B and T/M were calculated. Eight patients with breast cancer and a case of fibroadenoma showed intense accumulation of 201TlCl in the tumors. The T/B ratio was 1.20 +/- 0.68 and the T/M ratio was 0.68 +/- 0.31 in the 9 cases. Lymph node metastasis was detected in 2 of 6 cases that were confirmed at operation. No remarkable accumulation of 201TlCl was seen in 4 patients with benign process. One patient with benign tumor showed a false positive result. The rates of accuracy of mammography and ultrasonography for the same subjects were 82% and 84%, respectively. The results suggest that 201TlCl SPECT might be useful to assess breast cancer in cases in which the findings of other modalities are equivocal.