Objective: To evaluate the efficacy and toxicity of concurrent chemoradiotherapy (CRT) in multiple primary cancers (MPC) of the upper digestive tract in esophageal squamous cell carcinoma (ESCC). Methods: In a screening of 1193 consecutive patients diagnosed with ESCC and received radiotherapy, 53 patients presenting synchronous MPC in the upper digestive tract were retrospectively investigated. 53 consecutive patients with esophageal non-multiple primary cancer (NPC), matched by stage, age and sex, served as control. All of the patients received concurrent CRT. The median radiation dose was 60 Gy. Chemotherapy regimens were based on platinum and/or 5-fluorouracil. Clinical outcomes and treatment toxicities were compared. Results: Clinic-pathologic characteristics were well balanced between groups. MPC mostly located in esophagus (43, 81.8%), followed by hypopharynx (8, 15.1%) and stomach (2, 3.8%). In MPC and NPC patients, 94.3% and 96.2% completed the intended treatment. The immediate response rate was 73.6% vs 75.5%, with complete response rate of 11.3% vs 24.5% and partial response rate of 62.3% vs 51.0%. Two-year overall survival (OS), progression-free survival (PFS), locoregional progression-free survival (LRPFS) and distant progression-free survival (DPFS) were 52.2% vs 68.9% (p= 0.026), 32.9% vs 54.0% (p= 0.032), 60.8% vs 87.8% (p= 0.002) and 64.0% vs 70.8% (p= 0.22), respectively. Acute grade 3-4 toxicities were observed in 64.2% vs 54.7%, significantly higher in radiation esophagitis (49.1% vs 28.3%, p< 0.001), and mucositis (11.3% vs 00p= 0.027). Conclusions: Compared with matched NPC, ESCC accompanied with synchronous MPC was related to significantly impaired survival, elevated risk of locoregional disease progression and higher incidence of severe esophagitis and mucositis, following concurrent chemoradiotherapy. Future study on reasons for decreased efficacy of chemoradiotherapy will help to optimize treatment. Advanced radiation techniques may play a role in protecting normal tissues and reduce acute toxicities.