Genetic factors are probably more important in the etiology of gonadal neoplasia than generally appreciated. Seventeen familial aggregates of testicular germ cell tumors have been reported. These include concordantly affected twins, affected sibs of nonconsanguineous parents, and affected individuals in more than one generation. In a given family, affected members often had tumors of different histology, suggesting that the factors that cause one type of germ cell tumor may be the same as those that cause another type. Such an hypothesis is consistent with observations that many testicular germ cell tumors contain more than one cell type. Several familial aggregates of ovarian tumors have been reported. Germinal epithelial tumors have been reported in multiple sibs as well as in individuals in more than one generation. The genetic data suggest that either some ovarian epithelial tumors result from a mutant gene(s), probably autosomal dominant or X linked dominant, or polygenic or multifactorial factors play a role in the etiology of ovarian epithelial neoplasia. A few familial aggregates of teratomas (dermoid tumors), a germ cell tumor, have also been reported. The relatively high frequency of bilaterality and the relatively young age of onset also suggest that hereditary factors are important in teratomas. Familial aggregates of sex cord mesenchyme tumors have also been reported. Granulosa cell tumors and certain other ovarian tumors are often associated with the Peutz Jeghers syndrome.