PURPOSE: Several management options are available to patients with stage I seminoma, including adjuvant radiotherapy, surveillance, and adjuvant chemotherapy. We performed a pooled analysis of patients from the four largest surveillance studies to better delineate prognostic factors associated with disease progression. PATIENTS AND METHODS: Individual patient data were obtained from each center (Princess Margaret Hospital, Danish Testicular Cancer Study Group, Royal Marsden Hospital, and Royal London Hospital) for 638 patients. Tumor characteristics (size, histologic subtype, invasion of rete testis, and tumor invasion into small vessels [SVI]) as well as age at diagnosis were analyzed for prognostic importance for relapse. RESULTS: With a median follow-up of 7.0 years (range, 0.02 to 17.5 years), 121 relapses were observed for an actuarial 5-year relapse-free rate (RFR) of 82.3%. On univariate analysis, tumor size (RFR: 4 cm, 76%; P =.003), rete testis invasion (RFR: 86% [absent] v 77% [present], P =.003), and the presence of SVI (RFR: 86% [absent] v 77% [present], P =.038) were predictive of relapse. On multivariate analysis, tumor size ( 4 cm, hazard ratio 2.0; 95% confidence interval [CI], 1.3 to 3.2) and invasion of the rete testis (hazard ratio 1.7; 95% CI, 1.1 to 2.6) remained as important predictors for relapse. CONCLUSION: We have identified size of primary tumor and rete testis invasion as important prognostic factors for relapse in patients with stage I seminoma managed with surveillance. This information will allow patients and clinicians to choose management based on a more accurate assessment of an individual patient's risk of relapse. In addition, it will allow clinicians to tailor follow-up protocols based on risk of occult disease.