Objective. To verify whether sleep duration was related to body mass index z scores (zBMI) and bedtimes or ethnicity were moderators of the sleep duration-zBMI association among preschoolers from low-income families. Methods. 228 African-American and Hispanic parents and their preschoolers were recruited from Head Start Centers. Parents reported their preschoolers’ sleep duration and bedtimes using the Children’s Sleep Habits Questionnaire and their television watching. Food intake was measured using multiple days of digital photography to estimate energy intake at dinner. The Parenting Stress Index and the Center for Epidemiologic Studies Depression Scale were used to measure stress and depression. Parents’ and preschoolers’ height and weight were measured by trained staff. Results. Longer sleep duration was significantly associated with lower zBMI when controlling for demographics, bedtimes, energy intake at dinner and television watching, but not when also controlling for parents’ demographics, BMI, stress and depression. Preschoolers’ sleep duration-zBMI association was not moderated by bedtimes or ethnicity. Conclusions. Longer sleep duration was associated with lower zBMI among low-income preschoolers when controlling for preschoolers’ characteristics but this was no longer the case when also controlling for parents’ characteristics. Additional studies are needed on the moderators of the sleep duration-zBMI association among low-income preschoolers.