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Although the prevalence of obesity has markedly increased among US adults, health risks vary according to the severity of obesity. Persons with class 3 obesity (body mass index [BMI] ≥40) are at greatest risk, but there is little information about this subgroup.To examine correlates of class 3 obesity and secular trends.Adults (aged ≥18 years) in the United States who participated in the Behavioral Risk Factor Surveillance System telephone survey between 1990 (75 600 persons) and 2000 (164 250 persons).Body mass index calculated from self-reported weight and height.The prevalence of class 3 obesity increased from 0.78% (1990) to 2.2% (2000). In 2000, class 3 obesity was highest among black women (6.0%), persons who had not completed high school (3.4%), and persons who are short. During the 11-year period, the median BMI level increased by 1.2 units and the 95th percentile increased by 3.2 units.The prevalence of class 3 obesity is increasing rapidly among adults. Because these extreme BMI levels are associated with the most severe health complications, the incidence of various diseases will increase substantially in the future.