Objective. The Look AHEAD (Action for Health in Diabetes) study is a 16-centre randomised clinical trial in overweight and obese individuals with type 2 diabetes, designed to evaluate the long-term effects (up to 11.5 years) of intensive weight loss intervention on the time to incidence of major cardiovascular events. Research design and methods. Eligibility requirements are diagnosis of type 2 diabetes (determined by self-report and verification) in individuals aged 45-74 years and body mass index (BMI) > 25 kg/m2 (> 27 kg/m2 if currently taking insulin). The intensive lifestyle intervention is designed to achieve and maintain weight loss through decreased caloric intake and increased physical activity. The study is designed to provide 90% probability of detecting an 18% difference in major cardiovascular disease event rates in patients randomised to the intensive lifestyle intervention compared to the control group receiving standard diabetes support and education. Results. The 5,145 participants who were randomised between 2001 and 2004 were 63.3% white, 15.6% African-American, 13.2% Hispanic, 5.0% American Indian and 1.0% Asian-American, which closely paralleled the ethnic distribution of diabetes in the National Health and Nutrition Examination Survey (NHANES) 1999-2000 survey. Their average age at entry was 59± 6.8 years (mean ± SD), and 60% were women. There were 31.5% between 45-55 years of age, 51.5% were 56-65, and 17.0% were 66-76 years of age. Some 15.4% of participants were taking insulin at the time of randomisation and 14.0% had a history of cardiovascular disease. More men (21.3%) than women (9.2%) had a history of cardiovascular disease. Few participants (4.4%) were current cigarette smokers, compared to 16.2% in the NHANES 1999-2000 survey. Furthermore, 65.0% of participants had a first-degree relative with diabetes. Overall, BMI averaged 36±5.9 kg/m2 at baseline, with 83.6% of the men and 86.1% of women having a BMI > 30 kg/m2 and 17.9% of men and 25.4% of women having a BMI > 40 kg/m2. Conclusions. The Look AHEAD study has successfully randomised a large cohort of participants who have type 2 diabetes with a wide distribution of age, obesity, ethnicity and racial background and will examine the effects of lifestyle intervention on the incidence of major cardiovascular events.