Ultrastructural examination of untreated and estrogen-treated human testes revealed for the first time five types of paracrystalline inclusions in the cytoplasm of Leydig cells. The distinctions between each type of inclusion were based on longitudinal and cross-sectional ultrastructure. The inclusions were not found in Leydig cells containing Reinke crystals, nor were different types of inclusions found in the cytoplasm of the same cell. One form of inclusion or Reinke crystals were occasionally found in the nucleus. Some Leydig cells with intranuclear inclusions contained the same or a different type of inclusion in the cytoplasm. The present study revealed five types of paracrystalline inclusion in contrast to the one or two types described in previous reports. A possible explanation for this difference is that earlier investigators based their observations on small pieces of tissue obtained from biopsy, while the present data were obtained by studying many regions of each testis. All five types of paracrystalline inclusions were found in relatively normal as well as azoospermic (estrogen-treated) testes.