Telomere maintenance plays an important role in cell proliferation and tumor survival. Human male germ cells, which carry long telomeres and express telomerase, give rise to a highly heterogeneous group of malignant tumors. We compared telomeric length and telomerase activity between two major histological types of primary testicular germ cell tumors. Fifteen out of 16 seminoma samples revealed telomeric restriction fragment (TRF) length below 13 kb; the remaining seminoma showed a major TRF fraction of 18 kb and a distinct minor fraction of above 23 kb length. In contrast, all 13 samples from nonseminomas showed TRF length >/=23 kb, which is similar to that reported in human sperm. Nine out of 11 seminoma specimens and six out of seven nonseminomas studied showed moderate to high telomerase activity, the only telomerase-negative nonseminoma being pure mature teratoma. These results indicate to a major difference in telomeric length between seminomas and nonseminomas, which is apparently unrelated to the presence of telomerase activity, and suggest a germline-like homeostasis of telomeric length is preserved in human nonseminomas. Oncogene (2000) 19, 4075 - 4078.