The monoclonal antibody PC10 is used for immunohistochemical staining of the proliferating cell nuclear antigen (PCNA). The percentage of PC10-positive cancer cells is defined as the PC10 index. We evaluated the relationship between the PC10 index in pretreatment endoscopic biopsies and the prognoses of 47 patients with Stage II-III oesophageal squamous cell carcinoma treated with radiotherapy. The patients with a PC10 index > 40% had significantly poorer prognoses than the other patients (p = 0.0007). Proportional hazards model analysis indicated that only the PC10 index was a prognostic factor (p = 0.0009). The patient group of complete responders showed significantly lower PC10 indices compared to patients with a partial response or no change (p = 0.049). The PC10 index can be a good predictive indicator of the prognosis in patients with Stage II-III oesophageal cancer treated with radiotherapy.