Technical Abstract: OBJECTIVEdRates of severe obesity (BMI$40 kg/m2) are on the rise, and effective treatment options are needed.We examined the effect of an intensive lifestyle intervention (ILI) on weight loss, cardiovascular disease (CVD) risk, and program adherence in participants with type 2 diabetes who were severely obese compared with overweight (BMI 25 to ,30 kg/m2), class I (BMI 30 to ,35 kg/m2), and class II (BMI 35 to ,40 kg/m2) obese participants. RESEARCH DESIGN AND METHODSdParticipants in the Action for Health in Diabetes (Look AHEAD) trial were randomly assigned to ILI or diabetes support and education (DSE). DSE participants received a less intense educational intervention,whereas ILI participants received an intensive behavioral treatment to increase physical activity (PA) and reduce caloric intake. This article focuses on the 2,503 ILI participants (age 58.6 6 6.8 years). RESULTSdAt 1 year, severely obese participants in the ILI group lost29.0467.6% of initial body weight, which was significantly greater (P , 0.05) than ILI participants who were overweight (27.43 6 5.6%) and comparable to class I (28.72 6 6.4%) and class II obese (28.64 6 7.4%) participants. All BMI groups had comparable improvements in fitness, PA, LDL cholesterol, triglycerides, blood pressure, fasting glucose, and HbA1c at 1 year. ILI treatment session attendance was excellent and did not differ among weight categories (severe obese 80% vs. others 83%; P = 0.43). CONCLUSIONSdSeverely obese participants in the ILI group had similar adherence, percentage of weight loss, and improvement in CVD risk compared with less obese participants. Behavioral weight loss programs should be considered an effective option for this population.