Although obesity is a key component of type 2 diabetes, particularly in the Western world, many patients are not overweight by traditional criteria. However, evolving definitions of overweight and obesity and an appreciation of the factors influencing obesity in type 2 diabetes are redefining the concept of the non-overweight (lean) patient. Several factors need to be considered, including body mass index, waist circumference and ethnicity when considering whether a patient is lean, and appropriate obesity threshold values need to be used. Lean patients are more likely to be older at diagnosis, possibly have an immune component and may have a tendency towards certain pathophysiological characteristics, notably less insulin resistance and poorer insulin secretory capacity. However, pathophysiological heterogeneity may also be greater among lean patients and this imparts a particular challenge to therapy. An early focus on multiple metabolic defects may provide an optimal approach to treating the lean patient with type 2 diabetes in clinical practice, and combination therapy that addresses both insulin secretory dysfunction and insulin resistance would seem a rational approach.