BACKGROUND : Metachronous cancer in patients with head and neck cancer (HNC) is common and is associated with a poor prognosis. We aimed to evaluate the incidence of metachronous cancer at different sites according to age at diagnosis of index HNC. METHODS : We collected data on 2011 patients with oral cancer, oropharynx cancer, hypopharyngeal cancer, and laryngeal cancer as index cancers using the Osaka International Cancer Institute Cancer Registry database between 2005 and 2016. Among these, we analyzed 1953 patients after excluding 5 patients who were not followed-up and 53 patients with simultaneous multiple index cancers. We evaluated the cumulative incidence of metachronous cancer in the esophagus, lung, and other sites according to age at diagnosis of the index HNC using the Kaplan-Meier method. Multivariate logistic regression analysis was performed to identify factors that influenced the incidence of metachronous cancers following HNC. RESULTS : The cumulative incidence of metachronous esophageal cancer in young patients (< 65 years) was significantly higher than that in old patients (≥ 65 years) (12.1% vs 8.5% at 5 years, and 16.5% vs 11.2% at 10 years; p = 0.015). On the other hand, the cumulative incidence of the other cancers in young patients was significantly lower than that in old patients (7.8% vs 12.2% at 5 years, and 13.9% vs 15.3% at 10 years; p = 0.017). The cumulative incidence of lung cancer was not significance according to age at diagnosis of the index HNC. In the multivariate analysis, histological type (squamous cell carcinoma) and lesion location (hypopharynx and larynx) were independently associated with metachronous cancers. Moreover, age at diagnosis of the index HNC (< 65 years), histological type (squamous cell carcinoma) and lesion location (hypopharynx) were significant predictors of metachronous esophageal cancer incidence and lesion location (hypopharynx) was a significant predictor of metachronous lung cancer incidence. CONCLUSIONS : Risk stratification of metachronous cancers with age and other predictors may help to properly manage patients with HNC. BACKGROUND : The present study is a non-intervention trial.