Unlike the general population, among hemodialysis patients body mass index (BMI) is related to blood pressure (BP) and mortality inversely. To explore the reasons for this risk-factor paradox, the cross-sectional association of obesity with the following factors was examined: the prevalence of hypertension, its control, and echocardiographic left ventricular mass index (LVMI). Longitudinal follow-up explored the relationship of BMI with all-cause mortality. Furthermore, it explored whether poorer survival in leaner individuals was related to either high BP or greater LVMI. Among 368 hemodialysis patients, both the prevalence of hypertension and its poor control were inversely related to BMI. BMI was also inversely associated with evidence of excess extracellular fluid volume, but adjustment for this variable did not completely remove the inverse relationship between BP and BMI. Over 1122 patient-years of cumulative follow-up (median: 2.7 years), 119 patients (32%) died. In the first 2 years of follow-up, the mortality hazard for the lowest BMI group was increased; thereafter, the survival curves were similar. Adjusting for several risk factors including BP and LVMI did not remove the inverse relationship of BMI with mortality. In conclusion, leaner patients on dialysis have a higher prevalence of hypertension, poorer control of hypertension, more LVMI, and greater evidence of extracellular fluid volume excess. However, volume explains the greater prevalence or poorer control of hypertension only partially. Leaner patients have an accelerated mortality rate in the first 2 years; this is not completely explained by BP, LVMI, or other cardiovascular or dialysis-specific risk factors.