To examine the association between excessive prepregnancy weight and adverse outcomes, with a focus on women weighing over 200 lbs (91 kg) before pregnancy. Data were from the 1998-1999 New York City births file for 213,208 singletons with information on prepregnancy weight. Five categories of prepregnancy weight were constructed and used to predict gestational diabetes, preeclampsia, cesarean delivery, very low birthweight, macrosomia, and treatment in the neonatal intensive care unit (NICU). Statistical adjustments were made for mother's age, race or ethnicity, marital status, education, parity, social risk (eg, smoking), initiation of prenatal care, health insurance, and infant's sex. Maternal prepregnancy weight was associated with several adverse outcomes. Women in the heaviest group (> 300 lbs or > 136 kg) had the highest adjusted odds ratios (OR) for gestational diabetes (OR 5.2), preeclampsia (OR 5.0), and cesarean delivery (OR 2.7) compared with women weighing 100-149 lbs (45-67 kg). Compared with the reference group, the heaviest women were more likely to have a macrosomic infant and an infant treated in the NICU (OR 4.2 and 1.9, respectively). Even among a subsample of women who did not have any diabetic or hypertensive diseases, excess weight significantly increased the likelihood of macrosomia and NICU treatment. Blacks were disproportionately represented in the two heaviest groups (49.8% of those weighing 200-299 lbs and 63.9% of those weighing over 300 lbs). In this population-based study of pregnant women, the adverse outcomes associated with excessive weight underline the urgency of weight loss interventions before pregnancy. The analysis also suggests that research is needed on rapidly growing racial or ethnic subgroups most at risk for obesity.