Obesity and hypertension are two closely associated conditions and obesity probably predisposes to hypertension. In this investigation we evaluated central, systemic haemodynamic and intravascular volume characteristics of 51 established hypertensive obese subjects subgrouped for degree of obesity. Subgrouping according to body mass index (BMI) was as follows: lean hypertensives, 17 subjects (BMI 23.00 +/- 0.45 Kg/m2); moderately obese, 19 subjects (BMI 26.92 +/- 0.30 Kg/m2); severely obese, 15 subjects (BMI 32.78 +/- 0.45 Kg/m2). Obese hypertensives were characterised by significantly increased cardiac output and total plasma volume (P less than 0.05) and by decreased total peripheral resistance (not significantly) and left ventricular ejection fraction (P less than 0.01). These changes appear to be related to the degree of obesity. Significant correlation coefficients between body mass index and cardiac output, total blood volume, total peripheral resistance and ejection fraction were also observed. In conclusion, established hypertension in the severely obese patient appears to be a separate haemodynamic entity.