Background/Aims: Clinical trials of older adults are increasingly common, but risks of serious adverse events (SAE) may vary. We describe the incidence of SAE in two randomized trials, one community-based and one nursing home-based. Methods: We performed a secondary data analysis from two randomized clinical trials at one academic health center and 21 nursing homes involving 200 sedentary community dwellers aged 70-89 years and 185 female nursing home residents aged 65 years or older. Interventions included structured physical activity to reduce mobility disability in the Lifestyle Interventions and Independence for Elders (LIFE) study and oral cranberry capsules to reduce bacteriuria plus pyuria in nursing home residents (CRANNY) trial. We measured SAE incidence per 100 person-years and incidence of protocol-related unanticipated SAE per 100 person-years in LIFE and CRANNY trials. Results: Mean age and proportion of patients with dementia in LIFE and CRANNY trials were 79.3 years and 86.4 years and 0% and 78%, respectively. There were 179 total SAE in LIFE including 8 (4%) deaths, and 116 total SAE in CRANNY including 33 (28%) deaths. SAE incidence was 33.7 (95% CI 27.2, 41.8) events per 100 person-years in LIFE and 69.4 (95% CI 49.1, 98.1) events per 100 person-years in CRANNY. No protocol-related unanticipated SAE occurred in either trial. Conclusions: The frequency and severity of SAE vary in older adults. While SAE are common in nursing home residents, protocol-related, unanticipated SAE are rare in nursing home residents and community dwellers. This finding can inform trial monitoring protocols.