Objective: Obesity is a worldwide and challenging problem, negatively affecting every aspect of health.While treatment seems obvious: to limit energy intake, this approach appears overwhelmingly unsuccessful.Limiting food intake alone leads to loss of lean body mass which can be prevented by combining it with (moderate) exercise, maintaining lean body mass and improving insulin sensitivity, but experience teaches that this approach is equally unsuccessful.Methods: A review of the literature and recent, unpublished studies on the role of testosterone in the etiology and potential treatment of obesity and insulin resistance.Results: Lower-than-normal testosterone levels in men lead to an increase in fat depots, particularly abdominal fat.This type of fat distribution is associated with the metabolic syndrome and its sequels: diabetes mellitus type 2 and cardiovascular disease.Intervention studies with testosterone in hypogonadal men show that this fat accumulation can be reversed, with a parallel increase in lean mass.In long-term studies, substantial weight loss occurs with testosterone treatment.At the same time, improvement of insulin sensitivity and of biochemical risk profiles of cardiovascular disease is observed.These relevant insights are largely unknown to medical professionals dealing with the metabolic syndrome.Conclusion: Diagnosing and treating testosterone deficiency in men suffering from the metabolic syndrome and (pre)diabetes shows promising results.