Patients with clinical stage I testicular seminoma usually receive elective para-aortic lymph node radiation after orchiectomy, which is effective in controlling subclinical microscopic disease. However, the majority of patients with clinical stage I seminoma do not harbor occult metastases and, therefore, do not require elective nodal treatment. Vascular space invasion by the primary testis tumor recently has been shown to be an important predictor of metastases in nonseminomatous tumors but no such information exists to date in pure seminoma. Therefore, patients with clinical stage I testicular seminoma were compared to clinical stage II to IV cancer patients with respect to the presence of several features of the primary tumor. Vascular space invasion was identified significantly less frequently in stage I cancer patients (17%, 5 of 29) than in those with stage II or greater disease (39%, 11 of 28, p equals 0.03, 1-tailed t test). Microscopic invasion of the tunica and rete testis, and necrosis also were identified slightly more frequently in the higher stage cancer patients. Of the 12 patients with a maximum tumor dimension of more than 6 cm. 9 (75%) were in the stage II or higher group. Patient age, symptom duration and presenting complaint were similar in the 2 groups. Many higher stage cancer patients did not exhibit aggressive histological characteristics and, therefore, the absence of these features cannot be used to select patients for surveillance. On the other hand, patients with clinical stage I tumors that exhibit vascular space invasion may have an increased rate of occult para-aortic lymph node metastases. Therefore, the presence of vascular space invasion may be a useful criterion for exclusion of patients from surveillance protocols. Confirmatory data are needed before a final recommendation can be made.