LBA4009 Background: Chemoradiotherapy (CRT) with 5-fluorouracil (5-FU) and mitomycin-C (MMC) is standard treatment for anal cancer. This trial addresses two questions: whether (i) replacing MMC with cisplatin (CDDP) improves the complete response (CR) rate, and (ii) two cycles of maintenance chemotherapy after CRT reduces recurrence. Methods: Between 2001 and 2008, 940 patients (pts) were recruited to a multicenter, randomized factorial trial. Pts received 5-FU (1,000mg/m2/day on d1–4 and 29–32), radiotherapy (RT) (50.4Gy in 28 fractions), and either MMC (12mg/m2, d1; n=471) or CDDP (60mg/m2 on d1 and 29; n=469). Pts were also randomized to receive maintenance therapy (n=448) 4 weeks after CRT (two cycles of CDDP and 5-FU weeks 11 and 14) or no maintenance (n=446). Maintenance randomization was not considered appropriate in 46 pts. Statistical power was ≥80% to detect a difference in the CR rate of 5% (CDDP vs MMC), and 30% reduction in recurrence (maintenance vs no maintenance). Results: Median age 58 yrs; 62% male, 38% female; tumor site - canal (81%), margin (15%); stage T1-T2 (50%), T3-T4 (43%); node negative (62%), positive (30%). Median follow-up was 3 yrs. The CR rate was 94% MMC and 95% CDDP (p=0.53). MMC pts had more acute grade 3/4 haematological toxicities (25 vs 13%, p<0.001) but this did not result in an increase in neutropaenic sepsis (3.1 vs 3.2%, p=0.93). Non-haematologic grade 3/4 toxicities were similar (61 vs 65%, p=0.22). Preliminary analysis shows no statistically significant difference in recurrence free survival (RFS) (HR 0.89, 95% CI 0.68, 1.18; p=0.42) or overall survival (HR 0.79, 95% CI 0.56,1.12; p=0.19) for the maintenance comparison. The number of pre-treatment colostomies not reversed were similar between treatments (18 MMC vs 14 CDDP, p=0.65, Maint/No maint, p=0.23) and only 9 disease-free pts had colostomies performed (5 MMC, 4 CDDP). Conclusions: ACT II is the largest trial conducted in anal cancer. High CR (95%) and RFS (75% at 3 yrs) rates were achieved with this CRT. This excellent outcome may have been influenced by the absence of a gap in the RT schedule. There was no difference in CR rates between MMC and CDDP or in RFS rates with or without maintenance chemotherapy. 5-FU, MMC with RT remains the standard of care. [Table: see text]