Vitamin A deficiency is a public health problem in the Northeast of Brazil. This study aimed to determine the prevalence of vitamin A deficiency and associated factors among preschool children in Teresina, Piauí, Brazil. We studied the serum retinol levels and socioeconomic and demographic characteristics of 631 children from 36 to 83 months of age. The statistical association between each of the above characteristics and serum retinol levels was investigated by univariate and multivariate linear regression analysis. Mean serum retinol was 1.21 mmol/L (95%CI: 1.17-1.25 micromol/L) and was independent of gender (p = 0.259). Prevalence of vitamin A deficiency (retinol < 0.69 micromol/L) was 15.4% (95%CI: 12.7-18.4), with a tendency to decrease with age. Acceptable but not adequate retinol levels (0.70 to 1.04 micromol/L) were observed in 29% of children (95%CI: 25.2-32.4). There were positive associations between retinol levels and age, income, vitamin A supplementation, and maternal schooling. Vitamin A deficiency appeared as a moderately serious public health problem, emphasizing the importance of strategies to combat this problem in the region.