Background Features of cancer cachexia adversely influence patient outcomes, yet few currently inform clinical decision-making. This study assessed the value of the cachexia index (CXI), a novel prognostic marker, in patients for whom neoadjuvant chemotherapy and surgery for oesophagogastric cancer is planned. Methods Consecutive patients newly diagnosed with locally advanced (T3–4 or at least N1) oesophagogastric cancer between 1 January 2010 and 31 December 2015 were identified through the West of Scotland and South-East Scotland Cancer Networks. CXI was calculated as (L3 skeletal muscle index) × (serum albumin)/(neutrophil lymphocyte ratio). Sex-stratified cut-off values were determined based on the area under the curve (AUC), and patients were divided into groups with low or normal CXI. Primary outcomes were disease progression during neoadjuvant chemotherapy and overall survival (at least 5 years of follow-up). Results Overall, 385 patients (72% men, median age 66 years) were treated with neoadjuvant chemotherapy for oesophageal (274) or gastric (111) cancer across the study interval. Although patients with a low CXI (men: CXI below 52 (AUC 0.707); women: CXI below 41 (AUC 0.759)) were older with more co-morbidity, disease characteristics were comparable to those in patients with a normal CXI. Rates of disease progression during neoadjuvant chemotherapy, leading to inoperability, were higher in patients with a low CXI (28 versus 12%; adjusted OR 3.07, 95% c.i. 1.67 to 5.64; P < 0.001). Low CXI was associated with worsened postoperative mortality (P = 0.019) and decreased overall survival (median 14.9 versus 56.9 months; adjusted HR 1.85, 1.42 to 2.42; P < 0.001). Conclusion CXI is associated with disease progression, worse postoperative mortality, and overall survival, and could improve prognostication and decision-making in patients with locally advanced oesophagogastric cancer.