Risk factors for Toxocara canis (T. canis) infection were evaluated in a prospective study of disadvantaged preschool children. In addition, the hypothesis that T. canis exposure is associated with lower intelligence was tested. Seropositivity was tested at 2 years, 3 years, and at 4 years 10 months (4-10). Intelligence was measured at age 4-10 by the Full Scale IQ of the Wechsler Preschool and Primary Scales of Intelligence (WPPSI). Pica and ownership of a dog were unrelated to seropositivity. Seropositive children had lower scores on the Mental Development Index (MDI) of the Bayley Scales of Infant Development at age 1 year (prior to likely exposure). They also had less favorable scores on a measure of the quality of childrearing. These findings suggest that, for disadvantaged children, lower initial intelligence and less advantageous child rearing are risk factors for T. canis exposure. Seropositive children also had higher blood lead levels, probably as a result of the common pathway of hand to mouth transmittal. Seropositivity at 3 years, at age 4-10, or, cumulatively, at any of the age 2, 3, or 4-10 assessments was associated with the WPPSI IQ after adjustment for sociodemographic factors. Exposure at age 4-10-years was significantly associated with reduced IQ scores (p = 0.030). However, when the age 1 year MDI score was controlled, the estimate became nonsignificant. We, thus, can neither confirm nor deny a relationship of T. canis and intelligence, but the importance of considering prior developmental status is emphasized.