OBJECTIVE To compare the body fluid status assessments provided by conventional bioelectrical impedance analysis (BIA) and vector BIA in moderate and severe obesity. RESULTS We studied 516 normotensive Caucasian women (mean age: 48 +/- 9.2 years), who were age-matched and divided into four groups on the basis of their body mass index (BMI): 99 normal weight women with a BMI of 19-25 Kg/m2; 228 preobese overweight women with a BMI of 25-30 Kg/m2; 132 women with class I-II obesity (BMI: 30-35 Kg/m2), and 57 women with class III obesity (BMI: 40-64 Kg/m2). Single-frequency (50 kHz) tetrapolar (hand-foot) bioelectrical impedance measurements were made, and total body water (TBW) and extracellular water (ECW) were estimated using conventional BIA regression equations. The RXc graph method was used for vector BIA, with the set of 327 women with a BMI of 19-30 Kg/m2 being adopted as the reference population. Mean vector displacement followed a definite pattern, with progressive vector shortening as the BMI increased, and along a fixed phase angle. This pattern indicates more TBW due to a greater soft tissue mass with average normal hydration. Short and downsloping vectors indicating fluid overload were more frequent in the group with class III obesity than in the group with class I obesity (19 vs 5%). The absolute values of TBW and ECW were significantly higher in the obese and overweight subjects than in those with normal weight subjects. TBW as a percentage of body weight was significantly lower in the obese subjects. CONCLUSIONS BMI influenced the impedance vector distribution pattern, which proved to be consistent up to a BMI of 64 Kg/m2. Obese women with an altered body composition can be identified and monitored using vector BIA.