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During the 10-year period (1987–1996) of our study, 26,255 patients with cancer were admitted to our clinic and, of these, 271 (1%) patients had multiple primary malignant tumors. Ninety-two (34%) patients had synchronous tumors (synchronous group), and 179 (66%) patients had metachronous tumors (metachronous group). The mean age at first diagnosis was higher in the former group. The ratio of men to women was 1.36 in the synchronous group and 0.74 in the metachronous group (p = 0.018). Smokers and drinkers were more common in the synchronous group. Breast cancer and lung cancer were most prevalent, and associations between head/neck and lung cancer and between breast and breast cancer were the most frequent associations in both the synchronous and the metachronous group. The frequency of aerodigestive tumors was higher and that of mesenchymal tumors was lower in the synchronous group than in the metachronous group. Localization in the medial region and in the head/neck was more frequent in the synchronous group than in the case of metachronous secondary tumors.