We have encountered many cases wherein the metastatic nest of esophageal squamous cell carcinoma occupied only a small space in the lymph nodes because of which computed tomography( CT) and fludeoxy glucose( FDG)-positron emission tomography( PET) could not detect the lymph node metastasis satisfactorily. The false-negative lymph nodes that were not detected by FDG-PET before surgery were smaller in diameter, rate of occupation, and area of occupation than the true-positive lymph nodes. The smallest area of the cancer nest in the true-positive group was 7.5 mm2, and therefore, it was reasonable to consider a 5-mm diameter area as the criteria for correct diagnosis by FDG-PET. Most of the false-negative lymph nodes with a large area of carcinoma were attached to the primary tumor; therefore, they could not be precisely identified. The detection of false-negative lymph nodes by FDG-PET was not precise because of increases in the quantity of stroma-like cells in poorly differentiated carcinomas and in fibrosis caused by neoadjuvant therapy in the lymph nodes.