Abstract Background Fatty liver disease is characterized by the accumulation of fat vacuoles inside of the hepatocytes. Non alcoholic fatty liver is associated with obesity, type 2 diabetes, dyslipemia, the intake of certain drugs and with the so-called metabolic syndrome. However, there is little information on the clinical relevance of this disorder as a healthcare problem in the general population, since the studies published generally include a limited number of patients and the diagnosis is established on the basis of clear biochemical alterations and liver biopsy. Methods/Design The aim of the study is the prevalence of non-alcoholic fatty liver disease in a general adult population by hepatic ultrasonography. A population-based, descriptive, transversal, multicentre study. Eighteen primary care centres of the north of Barcelona and the Maresme Areas of Healthcare Management attending an urban and semi-urban population of 360.000 inhabitants. A randomized sample of 786 subjects of 15 years or older were selected from the population and assigned to the participating centres according to the Primary Care Information System (SIAP): This population is practically the same as the general population of the area. The following determinations will be carried out in all the participants: hepatic ultrasonography to detect fatty liver, a questionnaire concerning liver diseases, alcohol intake, smoking and drug use, physical examination including abdominal perimeter and body mass index and biochemical analysis including liver function tests and parameters related to the metabolic syndrome and the HAIR score. Ultrasonographic diagnosis of fatty liver will be made according to established criteria (American Gastroenterology Association) and diagnosis of metabolic syndrome according to the criteria of the European Group for the Study of Insulin Resistance. Discussion This study will attempt to determine the prevalence of non alcoholic fatty liver disease, as well as, the factors most frequently associated with the presence of this disease to thereby achieve the most appropriate treatment and avoid the evolution of the disease.