Persons with serious mental illnesses (SMI) such as schizophrenia and bipolar disorder have an increased risk of obesity and related chronic diseases and die 10-20years earlier than the overall population, primarily due to cardiovascular disease. In the ACHIEVE trial, a behavioural weight loss intervention led to clinically significant weight loss in persons with SMI. As the field turns its attention to intervention scale-up, it is important to understand whether the effectiveness of behavioural weight loss interventions for people with SMI, like ACHIEVE, differ for specific subgroups.