A survey was made of second primary cancers among patients who were enrolled in a large case-control investigation of oral and pharyngeal cancer, hereafter called oral cancer, during 1984-1985 in four areas of the United States. Among the original 1090 patients with oral cancer (nearly all squamous cell carcinomas), 107 developed a second cancer (one-half of them squamous cell) by the end of follow-up in June 1989 (average follow-up 2.6 years), with 69% occurring in the oral cavity, pharynx, oesophagus, larynx or lung. Rates of second tumours varied by age and socioeconomic status, but not sex or race, and were higher among those whose initial cancer was localised, even after adjusting for their longer survival. Long-term survival was lower among those with second cancers. Conditional on surviving for 2 years, the survival at 5 years was under 50% and nearly 70%, respectively, for those with versus those without a second cancer in the first 2 years. These findings confirm the exceptionally high rate of second cancers (especially of the aerodigestive tract) following oral cancer, describe the clinical and pathological features of patients with multiple cancers and indicate the importance of preventive measures.