Background: Treatment options for clinical stage I seminoma include adjuvant radiotherapy (RT) as well as surveillance and adjuvant chemotherapy (CT). Data available from the surveillance, adjuvant RT and adjuvant CT series suggest that all treatment strategies achieve a complete cure in almost 100% of the patients with stage I testicular seminoma. The success of surveillance of patients with stage I nonseminomatous germ cell tumors and the establishment of curative CT for advanced disease have led to re-examination of the standard treatment approach. We report the results of a retrospective analysis of the prognostic factors for stage I testicular seminoma. Methods: We retrospectively reviewed the survey of 87 patients with stage I testicular seminoma who were admitted in an institution between January 1982 and January 2009. We examined if the following tumor characteristics could possibly predict the relapse: age at diagnosis, tumor size, pathological T (pT) stage, elevation of beta-hCG level and/or LDH level, invasion of the rete testis, vascular invasion and lymphatic invasion. The cause-specific survival rate was calculated using the Kaplan-Meier method. Results: During a median follow-up of 8.1 years (range, 1.2-25.7 years), 13 (14.9%) relapses were observed, with an actuarial 5-year relapse-free rate (RFR) of 87.9%. The univariate analysis showed that the following characteristics were not predictive of relapse: age at diagnosis (RFR: 89% [<36 years] vs. 88% [≥36 years]), tumor size (RFR: 90% [<5.6 cm(median)] vs. 85% [≥5.6 cm]), pT stage (RFR: 91% [pT = 1] vs. 84% [pT = 2 and pT = 3]), elevation of beta-hCG level (RFR: 89% [normal] vs. 87% [elevated]), invasion of the rete testis (RFR:90% [absent] vs. 80% [present]), vascular invasion (RFR:86% [absent] vs. 87% [present]), and lymphatic invasion (RFR: 88% [absent] vs. 87% [present]). The overall 5-year survival rate was 97%. Conclusions: Prognostic factors for relapse could not be identified in this retrospective analysis of the patients admitted in a single institution. To identify the prognostic factors for risk-adapted treatment for stage I testicular seminoma, we need to study a large sample population in Japan.