The epidemic rise in obesity has fuelled the current debate over its classification as a disease. Contrary to just being a medical condition or risk factor for other diseases, obesity is a complex disease of multifaceted aetiology, with its own disabling capacities, pathophysiologies and comorbidities. It meets the medical definition of disease in that it is a physiological dysfunction of the human organism with environmental, genetic and endocrinological aetiologies. It is a response to environmental stimuli, genetic predisposition and abnormalities, and has a characteristic set of signs and symptoms with consistent anatomical alterations. Excess adipose tissue increases the work of the heart and leads to anatomical changes in this organ. It alters pulmonary, endocrine and immunological functions, all with adverse effects on health. Some of the complications of obesity include cardiovascular disease, non-insulin-dependent diabetes mellitus, obstructive pulmonary disease, arthritis and cancer. Given the excess mortality, substantial morbidity and the economic toll of obesity, this is a disease that warrants serious attention by the medical community. Obesity's status and acceptance as a disease is pivotal in determining its treatment, reimbursement for treatment and the development of widespread interventions.