K-ras mutation in a tumour is a powerful negative predictor for treatment success. Identifying tumour K-ras mutation is complex, and could be simplified by an appropriate blood test. Clinical studies were identified in which K-ras mutation status was assessed in both blood and tumour to ascertain whether blood K-ras mutation is predictive of tumour K-ras mutation. Between 29% and 100% of patients with a tumour K-ras mutation in 11 studies presented the same mutation in peripheral blood. Only 5/272 patients presented blood K-ras mutation in the absence of the same tumour mutation, possibly due to sampling errors. K-ras mutation in blood appears to indicate K-ras mutation in tumour, while the absence of blood K-ras mutation does not prove lack of mutation in the tumour. This suggests that a blood test for the detection of tumour K-ras may be possible, and could direct cancer treatment strategies.