Background: The level of lymph node involvement is the most important factor in staging colorectal cancer without metastasis. Sentinel lymph node mapping identifies the node(s) that most accurately reflect the lymph node status of patients and intensive techniques that improve staging can be focused on these nodes. Objective: The aim of this study was to assess the efficacy of ex vivo sentinel lymph node mapping in the staging of colon cancer. Design: The selection of the cohorts was carried out on a prospective basis from September 2009 to April 2013, including all cases with no randomisation. Settings: Patients from the Alava University Hospital health region (Alava–Basque Country) in Spain. Patients: Study of 250 patients diagnosed prior to surgery with colon cancer without distant metastasis. A comparative study was also performed based on a control group of 170 patients staged with conventional techniques, involving a single slice and haematoxylin-eosin staining, from June 2006 to February 2009. Interventions: In these patients, we used ex vivo sentinel lymph node mapping with methylene blue, studying the sentinel nodes with multiple slices and immunohistochemical techniques as well as haematoxylin-eosin staining. Main outcome measures: The main outcome variable was change in staging after the Sentinel lymph node mapping. Results: The Sentinel lymph node identification rate was 98%, with 3.7% of false negatives. Upstaging occurred in 11% of cases compared to the group studied using conventional techniques. Limitations: The patients are not randomly selected and are compared with a retrospective series. Conclusions: Ex vivo Sentinel lymph node mapping with methylene blue accurately reflects the lymph node status of patients with colon cancer. This approach upstages patients classified as stages I and II by conventional techniques to stage III, indicating chemotherapy that may improve their prognosis.