Background: Immune checkpoint inhibitors (ICI), particularly PD-1 and PD-L1 antibodies, prolong survival in advanced renal cell (RCC) and urothelial cancer (UC) patients. However, they can cause a variety of serious irAE. We assessed frequency and onset of irAE at experienced centers, and evaluated whether baseline characteristics were associated with irAE. Methods: Pharmacy and clinic schedules identified RCC and UC patients treated with ICI at USC Norris (n = 51) and UCLA (n = 26) from 2014-2016. Prior radiation was defined as completion within 3 months before ICI. Fisher’s exact 2-tailed test was used to evaluate for associations of variables with irAE. Results: 77 patients median age 69 years (49-93) received ICI, 31 with RCC and 46 with UC; 36 received atezolizumab, 29 nivolumab, 9 pembrolizumab, 2 durvalumab, and 1 avelumab. 25 (32.5%) had irAE including pneumonitis (4%), myositis (3%), neurologic event (1%), hepatitis (8%), colitis (1%), rash (8%), endocrinopathy (6%), or hematologic event (1%). 11 (14%) required steroids, 3 (4%) required more intense immunosuppression (infliximab), 1 died of complications from irAE (pneumonitis). 5 were rechallenged with ICI after irAE resolution. Median time to onset of irAEs was 15 weeks; earliest onset of irAE (non-dermatologic) was 3 days after 1^st dose, latest onset was 112 weeks. Associations of irAE with variables are listed in the table. Conclusions: irAE were more common in our experience than in published datasets, especially endocrine events, and could occur after the first dose. There was an association between irAE and allergy to antibiotics as well as external beam radiation therapy (EBRT) within 3 months before ICI therapy.