Background: Successful initial treatment of oral and oro-phayngeal cancer has led to the emergence of second primary tumours (SPTs). Population data are meagre. Methods: Occurrence of multiple primary cancers following a malignancy in a head and neck site was computed using data from a population-based cancer registry covering a population of 14 million. Results: Among 59,958 subjects reported to the registry, 5.5% males and 3.6% females developed a second primary cancer. At the sites studied, a total of 2771 second primary cancers were found, compared with an expected number of 2341. The standardised incidence ratio (SIR) for contracting a new primary cancer was 1.14 (95% CI = 1.09-1.19) for men and 1.34 (95% CI = 1.24-1.44) for women. There was a significantly increased risk for a second cancer in most of the upper aerodigestive tract sites that are generally regarded as tobacco associated, with an SIR for subsequent oral cancer of 5.56 in men and 15.31 in women. Subjects first detected with a pharyngeal cancer experienced the highest SIR for a subsequent tumour. Excluding tobacco-associated sites, the risk of a subsequent cancer was not significantly raised in either sex (SIR 0.87 (95% CI 0.81-0.93) for men; SIR 0.99 (95% CI 0.90-1.09) for women). Conclusions: The relative risk for multiple primary cancer was higher in younger subjects, those detected with a head and neck cancer during the 1990s as compared with earlier decades of the study, and among patients who received radiotherapy for their first tumour. By 20 years from the time of the first head and neck cancer, we estimate that approximately 30% of male patients and 20% of female patients will have developed an SPT.