The current epidemiologic data on hyperhidrosis are scarce and insufficient to provide precise prevalence or impact estimates. We sought to estimate the prevalence of hyperhidrosis in the US population and assess the impact of sweating on those affected by axillary hyperhidrosis. A nationally representative sample of 150,000 households was screened by mailed survey for hyperhidrosis and projected to the US population based on US census data. Ascertainment of hyperhidrosis was based on a question that asked whether participants experienced excessive or abnormal/unusual sweating. The prevalence of hyperhidrosis in the survey sample was 2.9% (6800 individuals). The projected prevalence of hyperhidrosis in the United States is 2.8% (7.8 million individuals), and 50.8% of this population (4.0 million individuals) reported that they have axillary hyperhidrosis (1.4% of the US population). Only 38% had discussed their sweating with a health care professional. Approximately one third of individuals with axillary hyperhidrosis (0.5% of the US population or 1.3 million individuals) reported that their sweating is barely tolerable and frequently interferes, or is intolerable and always interferes, with daily activities. Hyperhidrosis affects a much larger proportion of the US population than previously reported. More than half of these individuals have axillary hyperhidrosis, in which sweating can result in occupational, emotional, psychological, social, and physical impairment.