Head and neck cancer (HNC) significantly impacts nutritional status because the tumor limits swallowing function. In this sense, it is important to monitor the nutritional status throughout the life of any individual. A multicenter case-control study was carried out to analyze the BMI at 30 years of age, two years before diagnosis and at the time of diagnosis of individuals with oral cavity, oropharynx, and larynx cancers. It was observed that a 5% reduction in BMI during the two years before enrollment was associated with an increased risk of the oral cavity (OR = 3.73), oropharyngeal OR = 5.25), and laryngeal (OR = 5.22). Reduced BMI of more than 5% over two years before diagnosis was associated with HNC. Weight loss remained significant at diagnosis, but it is not possible to exclude reverse causality since most cases are at an advanced stage. BMI monitoring of individuals at potential risk for HNC can promote early diagnosis and nutritional interventions for HNC.