BACKGROUND Squamous cancers of the upper aerodigestive tract (UADT) are related to the use of tobacco and/or alcohol, and in North America they are more common among the poor. They are usually locoregionally confined at diagnosis, and local treatment with surgery and/or radiation therapy is often curative. This study compares the incidence and survival of this group of diseases in Canada and the U.S., two North American neighbors with many cultural similarities but significant differences in their health care and social programs. METHODS To describe and compare the case mix, incidence, and outcome of squamous cancers of the UADT in Ontario, Canada, and the U.S., we used the Ontario Cancer Registry (OCR) and the Surveillance, Epidemiology, and End Results (SEER) registries in the U.S. to identify all cases of cancer with International Classification of Disease (ICD) codes 141, 143-9, 160-1, and a subset of 140, which were diagnosed between 1982 and 1994. ICD-O histology codes were placed into clinically relevant groupings, and ICD-9 site codes were grouped into sites as defined by the International Union Against Cancer and the American Joint Committee on Cancer. Age-adjusted incidence rates were calculated for each site. For the SEER registry, race specific incidence rates were also calculated. Observed and expected survival were plotted by site and registry, and from these, relative survival was calculated. Survival was compared during the first 5 years after diagnosis and during the next 5 years among patients who had survived the first 5 years. RESULTS Of the 16,577 and 42,990 cases identified in the OCR and SEER registries, respectively, squamous cancer was by far the most common histology (94.1% in OCR, 94.6% in SEER) and will form the main subject of this report. The distribution of squamous cancers by site, subsite, age, and gender were remarkably similar in the two populations. Overall, the incidence was about 17% higher in the U.S. than in Ontario, and this difference was seen for all sites except the nasopharynx, which was more common in Ontario. The higher incidence in the U.S. in part reflects the much higher rate for African Americans than for Americans of other ethnic backgrounds. During the first 5 years after diagnosis, when most deaths from UADT cancer occur, there was a significant relative survival difference in favor of the U.S. for cancer of the supraglottis, and in favor of Ontario for cancer of the oral cavity. There was a nonsignificant trend in favor of Ontario for cancer of the nasopharynx. Within the SEER population, for all sites except the nasopharynx, 5-year relative survival was considerably worse for African Americans than for Americans of other ethnic backgrounds. Examination of survival beyond 5 years after diagnosis for patients who had survived the first 5 years revealed that for all sites, the observed survival continued to diverge markedly from the expected survival. The excess mortality ranged from less than 20% for glottic and nasopharyngeal cancers to about 30−40% for oropharyngeal and supraglottic cancers. CONCLUSIONS Despite remarkable similarities in case mix between the two countries, UADT cancers were more frequent in the SEER population of the U.S. than in Ontario, and this was partly attributable to the much higher incidence among African Americans. Significant differences between the registries in 5-year survival were seen for several sites. African Americans with UADT cancers had much worse prognoses than did Americans of other ethnic backgrounds. Patients who survive their UADT cancer remain at a higher-than-expected risk of death even after they have been cured. Cancer 2000;88:1728–38. © 2000 American Cancer Society.