Circulating tumour cells in the blood of cancer patients are markers for a poor outcome (prognosis) in people with breast, colorectal, lung and prostate cancer. However, little data is available for renal cell carcinoma (RCC).This multicentre observational study looked at the link between the number of circulating tumour cells and survival in people with metastatic RCC who were treated with sunitinib or pazopanib as a first-line treatment following surgery. The researchers used the Cellsearch® system to count the tumour cells at four time points: the day before treatment started (baseline), day 28, day 56 and then at progression, or at 12 months if there was no progression.There were 195 patients in the study, nearly 4/5 were treated with sunitinib, and 1/5 with pazopanib. At baseline, nearly half of the patients had one or more circularing tumour cells per millilitre of blood. Thirty patients had three or more circulating tumour cells/ml blood and an average time to when the drug stopped working and the cancer started growing again (progression-free survival) of 5.8 compared to 15 months in the patients less than 3 circulating tumour cells/ml blood. Patients with three or more circulating tumour cells/ml blood had a shorter estimated overall survival time of 13.8 months compared to 52.8 months in those with less than 3 circulating tumour cells/ml blood.This study provided evidence that the presence of 3 or more circulating tumour cells/ml blood at baseline is associated with a significantly shorter survival in patients with metastatic RCC.