The relationship of low prepregnant body mass index with breastfeeding was investigated in 1272 women who delivered a term infant with birthweight > or = 2500 g at the San Paolo Hospital in Milan, Northern Italy. Underweight was defined using the Institute of Medicine's cutoff of 19.8 kg/m(2). Women were interviewed via telephone through 12 months postdelivery about breastfeeding practices. Education level (high versus low, odds ratio [OR], 1.41), primiparity (OR, 1.35), vaginal delivery (OR, 0.74), and birthweight of the infant (normal versus high, OR, 1.89) were associated with low, as opposed to normal, pre-pregnant body mass index. After adjustment for these confounders, no difference was found between underweight and normal weight women for initiation or duration of breastfeeding (mean adjusted difference, 0.4; 95% confidence interval [95% CI], -0.1 to 0.9 months) or exclusive breastfeeding (0.1 [95% CI, -0.1 to 0.3] months). Underweight mothers of healthy term infants may not be at increased risk for not initiating or shorter breastfeeding.