BACKGROUND: Climate change is a global health crisis. In most regions, heat waves are expected to become longer and more frequent and air quality is expected to worsen. Few physicians discuss climate and health with patients, and related guidelines are lacking. Our objective was to quantify the prevalence of risk factors for illness related to climate change in the U.S. ambulatory setting. METHODS: From the 2018 National Ambulatory Medical Care Survey, a national probability sample of nonfederal, ambulatory encounters, we identified adults with risk factors for illness related to heat or air pollution exposure. RESULTS: We found 91.4% of encounters involved a patient with at least 1 risk factor, while 46.7% had 2 or more. CONCLUSION: A high prevalence of patients with climate-related health risk factors exists in the ambulatory setting, representing a significant opportunity for evidence-based climate and health patient education and preventative care.