Patients diagnosed with oral and pharyngeal (OP) cancer have a substantial excess risk of second OP cancer, but risk quantification is still uncertain and scanty information is available on the absolute excess risk of second OP cancer. We considered the risk of second OP primary cancer in a population-based series of 3,092 first primary OP cancers registered between 1974 and 2003 in the Swiss Cantons of Vaud and Neuchâtel (total population of about 786,000 inhabitants). A total of 233 second OP cancers were registered, versus 7.4 expected, corresponding to a SIR of 31.7 (95% confidence interval (CI) 27.7-36.0). The SIR was 68.5 in the first year after diagnosis of the first primary, and declined thereafter, leveling around 20. The SIR was 30.7 when the site of first neoplasm was the oral cavity, 42.5 for the tongue and 28.1 for the oropharynx or hypo-pharynx. Corresponding values for topographies of second primaries were 28.8, 50.4 and 26.2. The cumulative risk of second OP cancer 15 years after diagnosis of first OP cancer approached 22% in men and 17% in women. The incidence of first primaries increased over 20-fold between age 30-39 and 70-79, whereas there was no rise with age for second neoplasms.