BACKGROUND AND AIM: A higher body mass index (BMI) appears to be reversely associated with mortality in dialysis patients. Moreover, although women have better survival in chronic kidney disease (CKD), this survival advantage is cancelled in dialysis. The association between BMI and mortality and the gender difference remain controversial in advanced CKD. METHODS: This study enrolled 3,320 patients (1,938 men and 1,382 women) from southern Taiwan who had CKD stages 3-5 with a BMI of 15.0-35.0 kg/m2. RESULTS: During a median 2.9-year follow-up, there were 328 (16.9%) all-cause mortality and 319 (16.5%) cardiovascular (CV) events and death in male patients, 213 (15.4%) all-cause mortality and 224 (16.2%) CV events and death in female patients. Compared with the reference BMI of 27.6-30.0 kg/m2 in an adjusted Cox model, lower-BMI groups in men, BMI 15.0-20.0 kg/m2 and 20.1-22.5 kg/m2, were associated with higher risks of all-cause mortality: hazard ratios (HRs) 3.19 (95% confidence interval [CI], 1.97-5.18) and 2.01 (95% CI, 1.29-3.14), respectively. Higher-BMI group in men, BMI 30.1-35.0 kg/m2, was associated with a higher risk of all-cause mortality: HR 1.72 (95% CI, 1.02-2.96). Likewise, lower- and higher-BMI groups in men were associated with a higher risk of CV events and death. In women, these associations between BMI and poor outcomes were not observed. CONCLUSIONS: In advanced CKD, there was a reverse J-shaped association between BMI and all-cause mortality, and a U-shaped association between BMI and CV outcomes in men. Neutral associations between BMI and poor outcomes were detected in women. Gender could modify the effect of BMI on mortality in patients with CKD.