Background: The standard post-orchiectomy treatment for UK patients (pts.) presenting with high-risk stage 1 NSGCTT is 2 cycles of bleomycin, etoposide (360mg/m²) & cisplatin (BE₃₆₀P) chemotherapy or surveillance with BEPx3 at recurrence. 111 (CRUK/09/011) investigates whether BE₅₀₀P x1 would achieve recurrence rates similar to BE₃₆₀P x2. Methods: Pts aged ≥16 yrs post-orchiectomy for vascular invasion positive stage 1 NSGCTT or combined seminoma + NSGCT, with normalised tumour markers, received 1 cycle of bleomycin30000IU day 1, 8 & 15, etoposide165mg/m² day 1, 2 & 3, &cisplatin50mg/m²day 1 & 2. All pts received a prophylactic antibacterial & GCSF. Pts were assessed for tumour markers q2m, q3m, q4m in years (yrs) 1, 2, 3, respectively and q6m in yrs 4 & 5. CT scan of chest, abdo and pelvis was performed at 6, 12, 24 & 60months (m) with chest x-ray at other visits. Toxicity was assessed using NCI CTCAE v3. Recurrences were independently reviewed. A sample size of 236 pts excludes a 2-yr recurrence rate of ≥5% if ≤6 recurrences were observed (80% power, 5% α). Results: Between 18/2/2010 and 31/7/2014 246 pts. were recruited at 33 UK centres; 54% with NSGCTT, 46% with combined GCT. Median follow-up was 39.1m (IQR 30.0-50.8). Four patients had malignant recurrences at 5, 8, 12 and 27m. 2 yr recurrence rate in 236 treated pts = 1.3% (95% CI: 0.4 to 4.0%). All 4 pts with recurrences were treated with 2^ndline chemotherapy +/- surgery, 3 are alive and free from disease, 1 died at 9m with refractory disease. In addition there were 3 non-malignant recurrences at 7, 10 and 13m with teratoma differentiated in retroperitoneal nodes (3, 4 and 4.4cm). All these pts are disease-free post RPLND surgery. 41% pts had grade 3-4 CTCAE at end of treatment (neutropenia: 31% and febrile neutropenia: 7%). The 2 yr overall survival is 99.2% (95% CI: 96.7, 99.8%). Conclusions: BE₅₀₀P is safely deliverable & 2 yr recurrence rate is similar to that seen with 2 cycles BE₃₆₀P. The adoption of BE₅₀₀P x1 as standard would reduce overall exposure to chemotherapy in this young pt population. 111 is the biggest formal, prospective trial to date investigating adjuvant BEPx1 in high-risk stage 1 NSGCTT. Clinical trial information: ISRCTN37875250.