Introduction Non-alcoholic fatty liver disease (NAFLD) is the most common liver disease worldwide.

Objectives To estimate NAFLD prevalence and transition probability through different severity stages in a population older than 18 years of age.

Methods The survey was conducted from July 2005 to January 2007 in Putignano, Bari. A systematic random sample of 2301 subjects from the list of records maintained by general practitioners was drawn. Subjects were asked to complete a questionnaire regarding sociodemographic characteristics and life styles, underwent anthropometric measures and liver echography. A sequential logit model was used to estimate the effect of the explanatory variables on the odds and probabilities of passing a set of transitions.

Results Overall prevalence of NAFLD was 24.3% (male 36.7%, female 14.2%); NAFLD steady raised with age. BMI and its interaction with age resulted significantly associated with the odds of transition from less to more severe NAFLD, especially in early stages. At 40 years old the transition probability to pass from no NAFLD to any form of NAFLD reached 35% and 8% among men and women respectively. The effect of BMI on the most severe stage of NAFLD is almost entirely the result of transition between absence to light NAFLD for subjects in the range 18–30 years old. A small effect of BMI was also seen in post-menopause women.

Conclusions The implications of this alarming and silent prevalence of NAFLD derived from the fact that this liver condition may progress to more severe liver disease.