Purpose: The goal of our analysis was to study pretherapeutic circulating 25-OHD plasma levels in patients with previously untreated advanced gastric cancer treated in the randomised controlled phase III Erbitux (cetuximab) in combination with Xeloda (capecitabine) and cisplatin in advanced esophago-gastric cancer (EXPAND) trial (NCT00678535) and to explore whether low 25-OHD plasma levels are associated with worse prognosis and may compromise the clinical efficacy of cetuximab. Methods: Six hundred thirty patients with available pretherapeutic 25-OHD plasma levels and treated with chemotherapy based on capecitabine and cisplatin, or chemotherapy and cetuximab, were included. The Cox proportional hazard regression model was used to analyse the association between low 25-OHD and survival in both treatment arms. Results: Majority of study patients were found to have severe vitamin D deficiency. No prognostic impact of 25-OHD plasma levels could be found in our patient cohort, and there was no indication of an interference of 25-OHD plasma levels and the efficacy of treatment with the anti-epidermal growth factor receptor monoclonal antibody cetuximab. Conclusions: Although majority of patients with advanced gastric cancer show hypovitaminosis D deficiency, there is no proof for a negative impact on survival or reduced treatment response. A prospective study is needed to investigate the potential benefit of vitamin D supplementation in this patient cohort during first-line chemotherapy. (C) 2019 Elsevier Ltd. All rights reserved.