...receive atezolizumab plus platinum-based chemotherapy (group A), atezolizumab monotherapy (group B), or placebo plus platinum-based chemotherapy (group C)....median progression-free survival in the intention-to-treat population was 8·2 months (95% CI 6·5-8·3) in group A and 6·3 months (6·2-7·0) in group C (stratified hazard ratio [HR] 0·82, 95% CI 0·70-0·96; one-sided p=0·007)...Median overall survival was 16·0 months (13·9-18·9) in group A and 13·4 months (12·0-15·2) in group C (0·83, 0·69-1·00; one-sided p=0·027)...Median overall survival was 15·7 months (13·1-17·8) for group B and 13·1 months (11·7-15·1) for group C (1·02, 0·83-1·24)...Addition of atezolizumab to platinum-based chemotherapy as first-line treatment prolonged progression-free survival in patients with metastatic urothelial carcinoma.