L-[methyl-11-C]methionine (11C-methionine) uptake of seven primary breast cancers, four soft tissue metastases of breast cancer, and three other breast lesions was studied by positron emission tomography (PET). 11C-methionine accumulation was assessed by calculating the standardised uptake value (SUV). The mean SUV for breast cancer was 8.5 +/- 3.3 (s.d.), while the maximal uptake in the liver was 12.4 +/- 1.6, in the bone marrow 5.8 +/- 0.7, and in the myocardium 3.4 +/- 0.6. All eight malignant tumours larger than 30 mm in diameter accumulated clearly 11C-methionine, whereas none of the three smaller cancers (from 12 to 15 mm in diameter) were visualised. Strong uptake of 11C-methionine was associated with a large S-phase fraction (SPF) measured with flow cytometry (r = 0.77, P = 0.01), and the non-visualised cancers had all a small SPF (less than 5.5%). One benign tumour (an abscess) accumulated slightly 11C-methionine. The results indicate that both primary and metastatic breast cancer can be effectively imaged with 11C-methionine by PET, and that the accumulation of 11C-methionine may correlate with the proliferation rate of breast carcinoma.