Between February 1979 and March 1985, 126 patients with clinical stage I non-seminomatous germ-cell testicular tumors were entered into a surveillance study after orchiectomy. Of this group, 36 (28%) have relapsed. The prognostic significance of 13 clinical, histopathologic, and biochemical factors has been analyzed. Vascular invasion and lymphatic invasion (LI) within the primary tumor, histology, and involvement of the epididymis and rete testis were significantly associated with an increased risk of relapse. However, multiple regression analysis showed that only histology and LI were significant, independent prognostic factors. These findings provide the basis for the consideration of adjuvant chemotherapy for patients with apparent clinical stage I testicular non-seminoma who are at high risk of harboring occult metastases.