Between August 1981 and December 1984, 85 consecutive patients with clinical stage I nonseminomatous germ cell tumors of the testis who were suitable for close observation entered a surveillance study after orchiectomy alone. All patients had unequivocally negative chest x-ray, bipedal lymph-angiography, and computerized tomography of the abdomen and pelvis, and normal levels of alpha-fetoprotein and human chorionic gonadotropin before entering the study. Patients were followed closely for 24 to 64 months (median 42 months) with regular chest x-rays, plain films of the abdomen for lymphangiography control, and serum determinations of alpha-fetoprotein and human chorionic gonadotropin but it was difficult to obtain computerized tomography scans of the abdomen at scheduled intervals for such a long period. Followup was closed December 31, 1986. At that date 62 patients (73 per cent) were continuously free of disease after orchiectomy alone and 23 (27 per cent) suffered relapse. The over-all occurrence rate of retroperitoneal relapses was 16.5 per cent and they usually were detected late, 4 to 36 months (median 10 months) after orchiectomy. Lung metastases were detected much earlier, 2 to 10 months (median 3 months) after orchiectomy. Alpha-fetoprotein and human chorionic gonadotropin elevations preceded the radiographic demonstration of metastases in 8 patients only (35 per cent) and in 1 they were the only sign of relapse. All but 1 patient with relapse were cured with chemotherapy and/or surgery, with an over-all survival rate free of disease of 98.8 per cent. Invasion of the epididymis, rete testis and spermatic cord, primary scrotal surgery, peritumor vascular invasion and embryonal carcinoma were associated with a higher risk for relapse but it was impossible to find clear-cut indications to select patients for adjuvant chemotherapy, retroperitoneal lymphadenectomy or no treatment. Furthermore, the followup of retroperitoneal nodes proved to be much more difficult than expected. Unilateral or modified retroperitoneal lymphadenectomy facilitates management of clinical stage I nonseminomatous germ cell tumors of the testis: only the chest and markers must be followed, the status of the retroperitoneal nodes is known immediately and antegrade ejaculation is preserved in the majority of cases.