Spermatogenesis is under the control of a complex endocrine and paracrine system, including estrogen receptor (ER) signaling. In many target cells, ER promotes the transcription of c-fos and other proto-oncogenes to regulate cell growth and differentiation. Thus, in this study we evaluated the expression of the proto-oncogene c-fos and the immunolocalization of c-fos, phosphorylated c-fos and ERbeta proteins in the human testis. Testis tissue samples were obtained from 12 men undergoing orchiectomy as adjuvant treatment for prostate cancer, and were stained by immunohistochemistry for c-fos, phosphorylated c-fos and ERbeta localization. Both forms of c-fos proteins were immunoreactive, mainly in germ cells (spermatogonia, spermatocytes and spermatids) and Sertoli cells, while ERbeta was primarily present in somatic cells (Leydig, Sertoli and myofibrillar cells). In addition, testicular biopsies obtained from infertile men with obstructive azoospermia/normal spermatogenesis (n=8) or non-obstructive azoospermia/severely impaired spermatogenesis (n=12) were evaluated for c-fos and ERbeta mRNA levels using real time polymerase chain reaction. The expression of c-fos mRNA was significantly lower (fold change = 0.08, p<0.05) whereas that of ERbeta mRNA was higher (fold change = 9.43, p<0.05) in the testis of men with non-obstructive azoospermia compared to those with obstructive azoospermia. These findings suggest a complex interrelation between estrogen signaling and c-fos transcriptional activity within the human testis, with the increase of ERbeta mRNA being putatively a compensatory mechanism for lower c-fos expression in infertile men with damaged spermatogenesis.