We retrospectively reviewed the genetic abnormalities detected clinically in 455 men with advanced germ cell tumors referred for chemotherapy. Of the patients 49 had extragonadal and 406 had testicular germ cell tumors. Of 19 patients with mediastinal germ cell tumors 4 (21 per cent, 3 with teratocarcinoma and 1 with endodermal sinus tumor) had Klinefelter's syndrome. Three of these patients had a 47XXY and 1 had a 48XXYY karyotype. No Klinefelter's syndrome was observed among 30 consecutive patients with retroperitoneal germ cell tumors or among the 406 with testicular tumors. Karyotypes of 35 consecutive patients with testis cancer without evident congenital abnormalities showed normal chromosomal patterns. We found 2 patients with Down's syndrome and testicular tumor, for an incidence of 0.5 per cent (probably significant). We also describe 2 cases of nonseminomatous testicular cancer and Marfan's syndrome (0.5 per cent incidence versus a 5 of 100,000 incidence of Marfan's syndrome in the general population). Apparently, genetic abnormalities are increased in men with germ cell tumors and we discuss the significancae of this association.