Carcinoma in situ (CIS) of the testis is the precursor lesion of most testicular germ cell tumors (TGCTs). Karyotyping of CIS is important for a better understanding of the pathogenesis of TGCTs and the progression to invasive cancer. We karyotyped three cases of CIS. All three cases showed a numerical abnormal chromosomal pattern. In one case, two copies of the germ cell tumor marker i(12p) were found.