Background: Trimodality therapy (TMT) with maximal TURBT followed by chemoradiation(CRT) is a standard of care for select patients with muscle invasive bladder cancer (MIBC). This trial evaluates the activity of atezolizumab (atezo) in MIBC in combination with TMT. This trial was designed with pre-specified safety analyses of the first 80 patients (40 in each arm). At the time of SWOG fall 2020 DSMC report deadline we had enrolled 84 patients but data on only 73 patients were available. The same data are being submitted to ASCO GU meeting. Methods: This trial is testing atezo every 3 weeks for 6 months given concurrently and adjuvantly with CRT vs. CRT alone in 475 patients with MIBC T2-T4aN0M0 disease. Patients are stratified based on PS; T2 vs T3 or T4; choice of chemotherapy; and radiation field (bladder only vs small pelvis). Patients undergo biopsy 3 months after finishing CRT to assess treatment response. Patients are followed for 5 years for recurrence or survival. This trial was not preceded by a phase I study but was designed with a safety run in of 80 patients. Study team agreed on the study design based on available data from other tumor types and initial experience from investigators running smaller similar trials. It was pre-specified that if we observe more than 25% patients having grade 3-5 colitis or cystitis in the atezo arm or any other toxicity which is deemed clinically significant and related to atezo, the trial investigators and DSMC would consider stopping further enrollment. Results: 36 patients were enrolled on the TMT alone arm and 37 patients on the TMT + atezo arm. No grade 3 or higher colitis was reported in the atezo arm. Only one patient had treatment related grade 3 radiation cystitis which was diagnosed after finishing atezo treatment. No steroids were given. Overall 23 grade 3 or higher toxicity events were reported in the atezo arm vs 11 in non- atezo arm. Most common toxicity was hematological which was considered non-immune related. None of the grade 3 or higher toxicities were considered to be immune related by the treating investigator. Conclusions: There is no evidence of increased immune related grade 3-5 AEs.DSMC has recommended to continue enrollment. Adverse Events with No Entries for Grades 3 to 5 Have Been Suppressed Clinical trial information: NCT03775265.