In oncology, a number of new potential therapeutic modalities, including gene targeting, are currently under investigation. To evaluate their response at a preclinical level, a non-invasive method providing information about cell proliferation would be highly valuable. The growth fraction can be assessed by the incorporation of thymidine into the DNA of S-phase cells. We report the use of the thymidine analogue bromodeoxyuridine (BrUdR) labelled with bromide-76 (76Br) in positron emission tomography (PET). PET scans using [76Br]BrUdR were performed in seven patients with metastatic melanoma. The in vitro cell proliferation in these metastases (n = 7) was compared with immunohistochemically evaluated cell proliferation using anti-bromo-deoxyuridine and MIB-1 antibodies after excision. Blood samples were taken to analyse the kinetics of the radiopharmaceutical. The accumulation of [76Br]BrUdR in PET correlated significantly with the immunohistochemically assessment of S-phase and cycling cells. In one patient a clinically unexpected metastases was found on [76Br]BrUdR-PET which became evident 4 weeks later. Analysis of blood samples showed a fast disappearance of [76Br]BrUdR; 30 min after injection free bromide was the main form of radioactivity, resulting in a high background activity. Assessment of cell proliferation using [76Br]BrUdR is hampered because of fast debromation and high background activity. The results are thus rather the effect of the increased circulation in more rapidly proliferating metastases than Incorporation of [76Br]BrUdR into proliferating cells.