Indium-111 octreotide and thallium-201 scintigraphic studies were compared in 21 patients (16 with palpable and five with non-palpable lesions) suspected of having breast malignancies on the basis of mammography. Early (15 min) and late (3 h) Tl-201(111 MRq) and 4-h and 24-h In-111-octreotide (111-148 MBq) static planar anterior images (matrix 256x256) were obtained on separate days. Images were evaluated both visually and quantitatively. Biopsy was performed following the im aging studies. Histopathology revealed 17 breast carci nomas (15 cases of invasive ductal carcinoma, one mucinous adenocarcinoma and one intraductal carcinoma) and four benign breast lesions (two fibroadenomas, one abscess and one case of fat necrosis. The means histopathological tumour size (mean largest diameter) was 3.38+/-1.9cm. In-111-octreotide detected 16 of the 17 breast cancers (94%) while Tl-201 detected 13 of them (76%). Both In-111-octreotide and Tl-201 missed one nonpalpable carcinoma showing only an isolated cluster of microcalcifications on mammography. The smallest tumour size detected by both agents 1.5x1.5 cm. Of the four benign lesions, only the breast abscess revealed both Tl-201 and In-111-octreotide uptake. In-111-octreotide scan also showed tracer uptake in five of the six patients with histologically proven axillary metastases, while four of these six patients showed Tl-201 uptake. The tumour/background (T/B) ratios of late In-111-octreotide and Tl-201 images were 1.71+/-0.38 and 1.46+/-0.30 respectively (P=0.039). In this preliminary study, In-111-octreotide yielded more favourable results than Tl-201 in the detection of breast carcinomas. However, the diagnostic efficacy of In-111-octreotide imaging needs to be investigated in larger patient series.