Evidence accumulated for the past two decades leads to the conclusion that obesity enhances the development of acute pancreatitis and worsens its clinical course. Is this true? We will try to give an answer to this issue by presenting the scientific data accumulated thus far. “Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy and/or increased health problems.“ (World Health Organization, 2000) The main problem with obesity is determining the best (and easiest) way to measure it. According to the definition, one should calculate the total amount of body fat a person has and deduct the “normal” amount of fat from it. Several methods have been developed, each with its strengths and weaknesses. (Kamel et al, 2000 ; Browning et al, 2011) Body mass index or BMI is the basic method used to determine obesity. It is a measure obtained by dividing the patient’s weight (in kilograms) with the square of his/her height (in meters) ; obesity is defined as BMI > 30 kg/m2. The method is based on the presumption that a person’s excess weight predominantly consists of fat. The advantage of this method is its application simplicity, namely the lack of complicated procedures needed to determine it as well as the fact that it has been globally accepted. The disadvantages are the consequences of the above mentioned presumption namely that a person’s excess weight predominantly consists of fat as well as the lack of body composition in the equation: a person who gains weight due to a component other than fat will have a falsely increased BMI, e.g. athletes have muscle hypertrophy ; patients with ascites (liver cirrhosis) and peripheral edema (renal failure, heart failure) accumulate water, etc. Other methods used to determine obesity measure the amount of subcutaneous fat tissue. These methods are based on the fact that the amount of subcutaneous fat tissue correlates well with the amount of excess fat tissue. The methods include the measurement of skin fold thickness, waist diameter and waist-to-hip ratio. As is the case for BMI, these methods are simple, requiring only a meter or a simple measuring instrument and the results are easily interpreted. The limiting factor for these methods is the presence of edema in the investigated areas (liver cirrhosis, heart and kidney diseases). The method that is not affected by the presence of excess water is dual-energy X-ray absorptiometry (DEXA). It is used to measure body composition based on the difference in the absorption of X-rays in different types of tissues (bone, fat, muscle, water). Compared to Acute Pancreatitis 36 other methods, DEXA is rather expensive, requires radiological equipment and a radiology specialist to interpret the results ; also, it uses radiation (X-rays), which makes it potentially harmful for the patients. After two decades of tedious work in finding the best method for estimating the amount of body fat in acute pancreatitis, scientists offer no clear answers. Although some data suggest that waist diameter and waist-to-hip ratio have the best correlation with the occurrence of complications in acute pancreatitis, BMI is still widely used as the standard procedure. The following sections offer a detailed insight into the best methods for estimating the amount of body fat in acute pancreatitis.