The ultrastructural features of Leydig cells in patients with high serum LH and low serum testosterone are described. The results are compared with the structure of Leydig cells in normal conditions and after treatment with human chorionic gonadotropin (hCG) according to data existing in the literature. The continuous overstimulation of Leydig cells by higher levels of LH apparently induces significant modifications in many of the morphological elements characteristic of these cells. However, these differences were not constantly present in the cases observed. The abundance and the large size of mitochondria were the only common characteristics in the patients studied.