Objectives. Cryptorchidism is associated with infertility, even in those patients with unilateral undescended testes. The mechanism for this infertility is not understood. We demonstrated recently that in mice, a stable nuclear matrix, a structural component of the nucleus that organizes DNA, is necessary for proper embryogenesis. We tested the hypothesis that spermatozoa from cryptorchid patients had unstable nuclear matrices. Methods. Semen samples from 7 patients with a history of undescended testes and decreased fertility were tested for sperm nuclear matrix stability using our halo assay. Results. All 7 patients were found to have unstable nuclear matrices, as compared with controls. Conclusions. This preliminary study suggests that one factor in the decreased fertility of cryptorchid patients may be unstable sperm nuclear matrices.