Considering the high morbimortality rate in oncologic surgeries of the gastrointestinal tract, especially in patients with malnutrition, the use of predictive tools is necessary, since preoperative strategies could improve postoperative outcomes. To evaluate body composition by computed tomography and its association with morbimortality post esophagectomy and total gastrectomy. Prospective cohort study (n = 80). Sociodemographic, diagnostic, treatment and postoperative data were collected. Anthropometric and biochemical (hemoglobin, transferrin, and albumin) data were evaluated. The muscle mass was calculated through two methods, the muscle mass index (MMI) and the psoas total area (PTA). For postoperative complications classification, the Clavien-Dindo scale was used. The prevalence of muscle depletion found was 33.8% by MMI and 61% by PTA (poor agreement, kappa = 0.25). Complication rates were 18.5% in gastrectomies and 50% in esophagectomies. No statistically significant difference was found between the presence of muscle depletion and complications. However, when stratified by surgery, a borderline association was found between the MMI and post esophagectomies complications (P = .05). Despite the high prevalence of muscle loss, it was not possible to correlate it with surgical outcomes for gastrectomies, but for esophagectomies, there may be relevance due to borderline association, although patients received nutritional therapy.