Background: The prognostic value of a low skeletal mass index (SMI) has been investigated in locally advanced oesophageal (LAE) cancer at diagnosis. However, nothing is known about its evolution and clinical impact between initial diagnosis and recurrence. Methods: A total of 89 patients treated for LAE cancer between January 2009 and December 2019 were included in this study. Computed tomography (CT) scans before treatment and at recurrence were evaluated. SMI and other body composition parameters were analysed by the L3 scan method. Results: Participants were aged 66.0 (36.0-86) years. The incidence of low SMI increased by 12.3% between diagnosis and recurrence (70.7% vs. 83.0%, respectively) over a median follow-up of 16.9 (1.7-101.6) months. Patients with high SMI at diagnosis showed loss of muscle mass (58.0 vs. 55.2 cm 2 /m 2 , respectively; P < 0.001) and decreased body mass index (BMI) (27.9 vs. 26.3 kg/m 2 , respectively; P = 0.05), but fat mass was increased (68.9 vs. 72.0 cm 2 /m 2 , respectively; P = 0.01). Patients with low SMI at diagnosis showed no significant changes in body composition parameters and no improvement of SMI, even with nutritional support. Low SMI (hazard ratio [HR]: 1.8; 95% confidence interval [CI]: 1.02-3.16) was an independent predictor (P = 0.041) of high nutritional risk index (HR: 1.79; 95% CI: 1.03-3.11; P = 0.039) at diagnosis. Conclusions: The percentage of patients with a low SMI increased during follow-up. Our data suggest that an assessment of skeletal muscle parameters and nutrition support may be more useful in patients with a high SMI.