Our previous studies have shown that transgenic male mice expressing human P450 aromatase (AROM+) are infertile. In the present study, we followed the testis phenotype up to 15 months of age in these mice. The testes of the old AROM+ mice showed Leydig cell hypertrophy and hyperplasia, as indicated by the staining for steroidogenic enzymes and androgen and estrogen receptors. However, the Leydig cell adenomas did not show signs of malignization. In contrast, we observed a marked increase in the number of activated macrophages in the testicular interstitium of the aging AROM+ mice. The macrophages were further shown to express high levels of CD68 (a monocyte/macrophage marker) and secrete TNFalpha, indicating strong activation, presumably by estrogen exposure. The increased activity of the macrophages was associated with Leydig cell depletion (analyzed at the age of 9 and 15 months) and an increased number of mast cells and fibrosis in the testicular interstitium. Interestingly, similar findings have been made in testes of infertile men. Hence, the aging AROM+ males present with a phenocopy of inflammation-associated infertility in men, providing a model for further studies on the putative link among estrogens, orchitis, and infertility.