The aim of this study was to characterise metastatic renal cell carcinoma in 18 patients with positron emission tomography (PET) utilising C-11-5-hydroxytryptophan, plasma biochemistry and neuroendocrine immunochemistry. Of these 18 patients, ten underwent the PET investigations. The standardised uptake values (SUVs) in hepatic deposits were higher than those in pulmonary lesions, with mean values of 3.15 and 2.35, respectively. The immunohistochemical study included staining of 10/18 surgical tumour specimens with antibodies reactive with chromogranin (Cg), neurone-specific enolase (NSE), and synaptophysin (Sy). In 17/18 patients, plasma measurements of the neuroendocrine peptides, CgA, CgB, pancreastatin, and serotonin, were performed. The results obtained in this study show that PET with C-11-5-hydroxytryptophan, a precursor in serotonin synthesis in neuroendocrine cells, can be utilised to visualise primary renal cell cancer and its secondaries in vivo. The results obtained also suggest that neuroendocrine differentiation may occur in human RCC.