Bioelectrical impedance analysis enables a rapid and safe assessment of body water. When it is assumed that the hydration factor of the fat-free mass is constant and is not different in the obese state, then fat-free mass and thus body fat can be assessed with bioelectrical impedance. However, several factors limit the valid application of bioelectrical impedance analysis in the severely obese state. One is the assumption of a constant hydration factor. Furthermore, body geometry is different in the obese state and body water distribution may also be different. All these factors have an effect on the validity of the method in the severely obese state, for which the amount of body fat generally will be underestimated with use of prediction formulas developed in normal-weight subjects. I discuss these limiting factors and provide some theoretical background.