To determine the correlation between pregestational body mass index and certain body composition parameters measured by bioelectric impedance (percentage of fat weight, fat mass, and lean weight) and to compare the body composition of women in immediate puerperal period with non-pregnant women, a cross sectional study was done including 86 women in physiologic puerperal period and 90 healthy non-pregnant women, from the Instituto Mexicano del Seguro Social, in Chihuahua, Mexico, aged 16 to 40 years old. The variables considered were the body mass index (BMI) before pregnancy, percentage of fat, fat mass, lean weight, total water and bio-resistance. Body composition was measured by bioelectric impedance (BIO) from 4 to 12 hours after delivery. Correlation coefFicient for BMI and percentage of fat was r = 0.66 (p < 0.01), and for BMI and fat mass, it was r = 0.74 (p < 0.01). In non-pregnant women the total water was 35.5 +/- 5 L and in women in the immediate puerperal period 38.5 +/- 5 L (p < 0.01). In conclusion, BMI before pregnancy is a suitable predictor for body composition in the puerperal period. However, it does not discriminate important variables such as total body water, so it may be convenient to use BIA for surveillance of body composition during pregnancy.