Carcinoma in situ of the testis is the precursor of seminomas and nonseminomatous germ cell tumors of the adult testis. It is a frequent finding in the testicular parenchyma adjacent to seminoma and nonseminoma. Thus far no differences have been demonstrated between carcinoma in situ adjacent to seminoma and nonseminoma; morphology, immunohistochemistry and ploidy are similar. Using in situ hybridisation of interphase nuclei, we demonstrate that the chromosomal constitution of carcinoma in situ adjacent to seminoma and nonseminoma is different. In particular, it appears that the number of copies of chromosome 15 is lower in carcinoma in situ adjacent to nonseminoma than in carcinoma in situ adjacent to seminoma, which is also true for the adjacent seminoma and nonseminoma.