This retrospective study was based on 9,089 patients and includes 855 cases (9.4%) of multiple primary cancers treated between 1975 and 1983. Three hundred fifty (42.1%) of these multiple cancers were considered synchronous, while 480 (57.9%) were classified as metachronous. Fifty percent of the metachronous cancers occurred within 31 months after the first primary; the mean time between the first primary and the second primary was 45 months. Approximately 22% of second primaries were found more than 5 years after the initial tumor. Five-year survival rates were higher in metachronous cancers (55%) than in synchronous cancers (18%). Survival rates varied according to the treatment. In 49.4% of synchronous tumors the treatment had to be modified because of the presence of two cancers. In these patients, 5-year survival rates were as low as 8%. In synchronous cancers that required no modification of the treatment, the prognosis was better and the survival rate was 28%. The preferential anatomical location of the second synchronous and metachronous cancers is discussed for first primaries situated in the oral cavity (103 cases), the oropharynx (120 cases), the hypopharynx (92 cases), and the larynx (87 cases). Second primaries in the esophagus were mostly synchronous cancers while, in the lung, second cancers were more frequently metachronous tumors.