Background: Patients (pts) with advanced UTC who fail first-line therapy have a poor prognosis and limited treatment options, with only modest benefit from chemotherapy. D (anti-PD-L1 antibody) is approved for the treatment of metastatic urothelial carcinoma after progression on platinum-based chemotherapy. Methods: Module A of the phase IIIb STRONG study (NCT03084471) will determine the short- and long-term safety of a fixed dose of D monotherapy (1500 mg, every 4 weeks) in pts with urothelial and non-urothelial carcinoma of the UT who progressed on or after prior chemotherapy. The primary endpoint is to determine the number of pts with adverse events of special interest (AESIs). This analysis included AEs with onset date on or after the date of first dose and up to 90 days after discontinuation of study treatment. Interim results from module A are reported. Results: A total of 700 pts received D monotherapy. Median age was 68.2 yr and 79.6% were male; 88.6% had an ECOG PS 0 or 1 and 11.3% an ECOG PS 2. Most (88.4%) had urothelial (transitional cell) carcinoma. Tumor PD-L1 expression was high (tumor or immune cell membrane positivity ≥25%) in 183/436 (42%) pts with PD-L1 data. Median treatment duration was 11.7 weeks (range, 1-73). AEs of any grade were reported in 86.9% of pts (Table), and in 89.0%, 86.9%, and 78.5% of pts with ECOG PS 0, 1, or 2, respectively. The most common grade ≥3 AESI was increased lipase (1.4%). Deaths related to study treatment occurred in 10 pts (1.4%). Conclusions: Fixed-dose D monotherapy has an acceptable safety profile as second-line therapy for UTC. Long-term safety data reported here are consistent with published studies of D monotherapy. Clinical trial information: NCT03084471