Obesity and its consequences are arguably the chief public health problems facing the developed world. Obesity causes many fatal diseases, in particular cerebrovascular and cardiovascular disease. Acanthosis nigricans (AN) is a common cosmetic disability in pigmented ethnic groups. It may present with periorbital darkening or darkening of the neck or knuckles as well as acrochordons (skin tags) around eyelids, neck or axillae. The more classical AN of axillae and groins is less often a cosmetic disability, although it is an important physical sign. AN is very common in pigmented populations throughout the world, irrespective of domicile. It is rare in whites. AN is closely associated with all the features of the insulin-resistance syndrome (IRS), especially obesity. AN and IRS share a similar prevalence and epidemiology. IRS (also known as syndrome X or the deadly quartet) is characterized by insulin resistance (IR) and its associated conditions, including obesity, dyslipidaemia, hypertension and diabetes mellitus (DM) type II. The sequelae of IRS are cardiovascular and cerebrovascular disease. The origins of insulin resistance and its sequel, IRS, are debated. Insulin resistance can result from obesity. Also obesity usually presents before AN, hypertension or DM II. For these reasons it may be more helpful to recognize instead an obesity syndrome. The obesity syndrome is characterized by a genetically determined thrifty metabolism that protects subjects in famine conditions but which, in conditions of plenty, leads to weight gain with all its consequences, including hyperlipidaemia, hypertension, IR with DM II, and, in due course, cerebrovascular and cardiovascular disease. In pigmented races, AN is an important early manifestation of the obesity syndrome. AN helps identify persons at particular risk of developing the obesity syndrome, dyslipidaemia, hypertension and IR with DM II. Recognition of AN, therefore, offers important opportunities for health screening and preventative medicine.