Studies suggest that children exposed to cigarette smoke in utero are at risk of becoming obese. Few researchers have evaluated the dose-response association between maternal smoking during pregnancy and childhood obesity or whether this association varies by maternal race/ethnicity. The authors obtained retrospective cohort data by linking records from the Pregnancy Nutrition Surveillance System and the Pediatric Nutrition Surveillance System on 155,411 low-income children born during 1995-2001 in 9 US states and 2 tribal nations. The authors examined maternal smoking status, duration of smoking, quantity of smoking, and both duration and quantity combined. Childhood obesity was based on a body mass index greater than or equal to the 95th percentile for sex and age, assessed at age 2-4 years. Maternal race/ethnicity modified the association between smoking during pregnancy and childhood obesity. Among non-Hispanic White mothers, both duration and quantity of smoking were positively associated with childhood obesity in a dose-response manner. Among non-Hispanic Black mothers, only heavy smoking was positively associated with childhood obesity. Among Hispanics, American Indians/Alaska Natives, and Asians/Pacific Islanders, smoking was not associated with childhood obesity. The inconsistent association between smoking during pregnancy and childhood obesity across race/ethnicity categories merits further investigation into potential explanations for this variation, which may include confounding, reporting bias, or unexplored biologic mechanisms.