Background: Concurrent CRT is standard treatment for patients with SCCA. We explored CRT compliance in ACT II, which compared 5FU/CisP with 5FU/MMC (wks 1 & 5) of a uniform RT dose (50.4Gy, 28 daily fractions (F) of 1.8Gy). Methods: We investigated the association between poor compliance and baseline factors (age, sex, site, T & N stage), type of CT (MMC/CisP) with progression free survival (PFS). Compliance was categorized as follows: RT, 5 groups: A = per protocol (50.4Gy in 28F in 38-42 days), B = ≤ 40Gy, C = 40-48.6Gy in 23-27F, D = 50.4Gy in > 42 days, E > 52.2Gy. CT, 2 groups: 1 = wks1 & 5 & 2 = wk1 only. Results: 933 and 862 of 940 pts were evaluable for RT & CT compliance respectively. Median follow-up was 5.1 yrs. Baseline characteristics of evaluable patients were similar to all 940 ACT II patients. Canal tumors, CisP, GFR<60 & WBC < 11 were borderline significant predictors of poor wk5 CT compliance (p 0.09, 0.07, 0.06 & 0.08 respectively). Poor CT compliance at wk5 impacted significantly on PFS (treatment adjusted HR: 1.63 (95% CI: 1.23-2.17), p = 0.001). No baseline factors analyzed, or chemotherapy type, were significant independent predictors of poor RT compliance. Conclusions: In ACT II poor CT & RT compliance (lower dose/prolonged OTT) adversely impacted on PFS. Treatment interruptions should be minimized and prolonged OTT compensated by hyperfractionation or possibly additional dose. Intensity Modulated RT may improve compliance. Patients with poor compliance to RT/CT may need closer monitoring following treatment. Clinical trial information: 26715889.