Early-stage head and neck carcinomas can usually be controlled with the appropriate treatment. In these patients, the long-term prognosis mainly depends on second metachronous malignancies, frequently in the aerodigestive tract. Our study aims to identify risk factors for the appearance of second tumours in this group of patients with early head and neck cancer. Of 949 patients included in the study, 189 (20%) developed a metachronous second primary malignancy, most frequently in the aerodigestive tract. Independent risk factors associated with second tumours were heavy alcohol use and the location of the index tumour in the oropharynx. Compared to non-drinkers, heavy drinkers (>80 g/day) presented a 1.8-times higher risk of a second tumour (CI 95%: 1.01-3.50). Patients with oropharyngeal tumours had a 2.15-higher risk than patients with oral cavity tumours (CI 95%: 1.03-4.47). Recursive partitioning analysis was used to characterise two risk groups for second tumours. The low-risk group included patients over 75 years and patients with low levels of carcinogen use. It comprised 171 patients (18%) with a 5.3% frequency of second tumours. The high-risk group accounted for 80% of the patients (n = 778), and the rate of second neoplasms was 16.3%. Classification of the patients according to the mentioned variables allows us to focus follow-up and prevention efforts on high-risk patients.