5020 Background: Three MRC randomised non-inferiority trials of adjuvant therapy for stage I seminoma, carried out between 1989 and 1991, have been updated to investigate the risk of late relapse and the method of detection of relapse. Methods: TE10 (JCO 17: 1146–1154, 1999) randomised 478 patients (pts) to 30Gy dog-leg (DL) or para-aortic (PA) irradiation (RT); TE18 (EORTC 30942, JCO 23:1200–1208, 2005) and TE19 collectively randomised 1,094 pts between 30Gy or 20Gy, with PA (89%) or DL(11%) RT at clinicians’ discretion. TE19 (EORTC 30982, Lancet 366: 293–300, 2005) alone compared RT with single-dose carboplatin (C) at AUC7. All trials required chest, abdominal and pelvic CT scans at 1, 2 and 3 yrs. Long-term follow-up (FU) data were sought; relapse-free rates (RFR) were determined by the Kaplan-Meier method, and hazard ratios (HR) from the Cox regression model (HR >1 favors standard treatment). Results: Median FU times, % followed to death or a documented minimum of 5 yrs, 5 yr RFR and HRs are as follows: TE10, 11 yrs, 80%, 96.1% (PA) and 96.2% (DL), HR 1.15 (90% CI 0.54–2.44); TE18, 7 yrs, 84%, 95.1% (30Gy) and 97.0% (20Gy), HR 0.59 (90% CI 0.35–0.99); TE19, 6.5 yrs, 78%, 5-yr RFR 96.0% (RT) and 94.7% (C), HR=1.25 (90% CI 0.83–1.89). A total of 106 pts relapsed after treatment, of whom 4 died of seminoma. Relapse sites differed by treatment, with abdominal relapses most common with C and pelvic most common with PA RT. 4 relapses occurred after 3 years (at 50 (C), 61, 64 and 91 months (RT)), all were disease free after chemotherapy. Details of the first sign prompting relapse diagnosis was available in TE19, and shows a difference between RT and C with 58% and 22% respectively detected by symptoms, and 17% and 61% respectively by CT; <2% of relapses were detected by the 3 yr CT. Conclusions: Long-term FU confirms the original conclusions of the 3 randomised trials. Relapse patterns differed between PA and DL RT and C. Abdominal CT is required for 2 years after C. Late relapse was rare and prolonged follow up after adjuvant treatment unnecessary Site and time of relapse by treatment Treatment Relapse site (% of total) Months to relapse patients relapses multiple abdomen pelvis other min med max PA RT 1470 57 23 8 34 36 4 16 91 DL RT 423 20 12 6 6 76 4 17 36 C 573 29 26 67 0 7 4 25 50 NB: combined data from 3 trials, rows do not reflect randomised arms Author Disclosure Employment or Leadership Consultant or Advisory Role Stock Ownership Honoraria Research Expert Testimony Other Remuneration Medical Research Council