We report a case of an acromegalic patient with a retroperitoneal seminoma secondary to a testicular tumor which had completely disappeared. Testicular ultrasonography did not show any pathological mass and histological examination indicated that the normal testis had been totally replaced by fibrous tissue, without neoplastic cells. Our patient had high levels of growth hormone (GH) and prolactin (PRL), both hormones with high immunostimulating effect, and an increase of interleukin-2 receptor-positive lymphocytes, a finding supporting enhanced immunological activity. Thus, we cannot exclude that the immunological pattern of this acromegalic patient may have played some role in the tumoral destruction.