The esophagus and the head and neck region, both having the squamous epithelium, are thought to be subject to the same carcinogenic factors. This report discusses the diagnosis and treatment of secondary cancer detected following the treatment of primary cancer in 21 patients who had metachronous cancer comprising esophageal cancer and head and neck cancer. The secondary cancer was already advanced when detected despite the fact that almost all patients were followed up regularly at the outpatient clinic of the facility where the primary cancer was treated. Although surgical treatment of the secondary cancer was similar to that of the usual primary cancer, the prognosis for the former was poor, indicating that the prognosis tends to be determined by the disease stage of the secondary cancer. Early detection of the secondary cancer requires esophagoscopy using the iodine dye method or close examination of the head and neck region at an otolaryngologic outpatient clinic at the time of treatment of the primary cancer and thereafter at 6- to 12-month intervals.