Baseline data from the Canadian National Longitudinal Survey of Children and Youth (NLSCY, a population-based study of child health, development and well-being), were used to determine biological, social and environmental correlates of poor development among preschool children. A total weighted sample of 1233 500 (n = 6982 unweighted) children aged from birth to 3 years were studied. Developmental attainment was measured by the motor, social and development (MSD) scale. Children scoring amongst the lowest 15% for their age group were categorized as having poor developmental attainment (PDA). Correlates of PDA were determined using logistic regression. The MSD scale may not be discriminatory enough to identify PDA in children aged