Although obesity is associated with increased risks of morbidity and death in the general population, a number of studies of patients undergoing hemodialysis have demonstrated that increasing body mass index (BMI) is correlated with decreased mortality risk. Whether this association holds true among patients treated with peritoneal dialysis (PD) has been less well studied. The aim of this investigation was to examine the association between BMI and outcomes among new PD patients in a large cohort, with long-term follow-up monitoring. Using data from the Australia and New Zealand Dialysis and Transplant Registry, an analysis of all new adult patients (n = 9679) who underwent an episode of PD treatment in Australia or New Zealand between April 1, 1991, and March 31, 2002, was performed. Patients were classified as obese (BMI of >/=30 kg/m(2)), overweight (BMI of 25.0 to 29.9 kg/m(2)), normal weight (BMI of 20 to 24.9 kg/m(2)), or underweight (BMI of <20 kg/m(2)). In multivariate analyses, obesity was independently associated with death during PD treatment (hazard ratio, 1.36; 95% confidence interval, 1.14 to 1.54; P < 0.05) and technique failure (hazard ratio, 1.17; 95% confidence interval, 1.07 to 1.26; P < 0.01), except among patients of New Zealand Maori/Pacific Islander origin, for whom there was no significant relationship between BMI and death during PD treatment. A supplementary fractional polynomial analysis modeled BMI as a continuous predictor and indicated a J-shaped relationship between BMI and patient mortality rates and a steady increase in death-censored technique failure rates up to a BMI of 40 kg/m(2); the mortality risk was lowest for BMI values of approximately 20 kg/m(2). In conclusion, obesity at the commencement of renal replacement therapy is a significant risk factor for death and technique failure. Such patients should be closely monitored during PD and should be considered for early transfer to an alternative renal replacement therapy if difficulties are experienced.