Peritoneal dialysis (PD) patients are classified as underweight, normal weight, or obese by height/weight indices including body mass index (BMI) and the body weight/desired weight (W/DW) ratio. We compared these classifications of degree of obesity in 378 women and 555 men on PD. We used these cut-off values: for underweight, BMI < or = 18.5 and W/DW < or = 0.9; for obesity, BMI > or = 30.0 and W/DW > or = 1.2. The W/DW values were calculated assuming first a small frame, then a medium frame, and finally a large frame for all subjects. Regardless of sex or skeletal frame, BMI correlated highly with W/DW (r value between 0.98 and 0.99); however, the range of BMI values corresponding by linear regression to the normal range of W/DW (0.9-1.2) was narrower than the range of "normal" BMI (18.5-30.0). Consequently, regardless of sex or skeletal frame, smaller fractions of the patient population were classified as underweight or obese by BMI standards than by W/DW standards. The degree of agreement of the classifications of subjects as underweight, normal weight, or obese by BMI and W/DW was evaluated by Cohen's kappa ratio. The kappa ratio varied between 0.47 and 0.58, indicating a reasonable--but not high--degree of agreement beyond chance. The highest kappa ratios were obtained assuming a medium skeletal frame for both women and men. Substantial discrepancies are observed in the classification of PD patients as underweight, normal weight, or obese by BMI and W/DW. Further research is needed to identify the height/weight index that has the strongest association both with clinical outcomes and with other, more precise measurements of body fat content.