This paper studies the relationship between family food insufficiency and being overweight in a population-based cohort of preschool children (n=2103) using data from the Longitudinal Study of Child Development in Québec (1998-2002) (LSCDQ). Family food insufficiency status was derived when children were 1.5 years of age (from birth to 1.5 years) and at 4.5 years of age (from 3.5 to 4.5 years). Children's height and weight were measured at home at 4.5 years. Overweight was defined according to the US CDC sex- and age-specific growth charts and Cole's criteria. Statistical analyses were done with SAS (version 8.2). In multivariate analyses, mean body mass index (BMI) was higher for children from food insufficient families compared to children from food sufficient families, even when important factors associated with BMI, such as child's birth weight, parental BMI, maternal education, and family income sufficiency were considered. We did not report any gender effects in the multivariate analyses. The presence of family food insufficiency at some point during preschool years more than tripled (OR 3.4, 95% CI 1.5-7.6) the odds for obesity using the Cole criteria, and doubled (OR 2.0, 95% CI 1.1-3.6) the odds for overweight at 4.5 years using the CDC growth curves indicator. We observed an interaction between birth weight and family food insufficiency in relation to being overweight at 4.5 years. Low-birth-weight children living in a household that experienced food insufficiency during preschool years are at higher risk of overweight at 4.5 years. Given this important finding, supportive interventions targeting low-income and food insufficient families, including pregnant women, are recommended for preventing overweight and obesity among their children.