In a search for consistent cytogenetic alterations in testicular germ cell cancers we have thus far studied some twenty surgical specimens of seminomas and nonseminomatous tumours. From the literature and our results it is now clear that such testicular tumours generally have a hyperdiploid to hypotriploid chromosomal content, and frequently possess a possibly site-specific chromosomal marker, an isochromosome 12p. A significant correlation between the presence of the i(12p) and advanced clinical stages has been revealed in our study. Several other chromosomal regions are consistently involved in cytogenetic changes: 1p and 1q, 6q, 7p, 9q, 12p, 17q, and 22q. Although there is little doubt that characteristic chromosomal lesions exist in testicular germ cell tumours, the impact which specific lesions may have on tumour progression is still unclear.