The ploidy of carcinoma in situ and invasive germ cell tumors of the adult testis was compared by DNA flow cytometry. Irrespective of the tumor type with which it was associated, the median DNA index of carcinoma in situ was about the same as that of seminomas and higher than the DNA index of invasive nonseminomatous germ cell tumors. These data indicate that seminoma and carcinoma in situ cells are not only phenotypically similar but also have the same ploidy.