OBJECTIVE: To investigate the prevalence of positive screening test for developmental delays in a cohort of children born in Pelotas, Brazil in 1993, and their risk factors. METHODOLOGY: A sample of 20% (1,363 children) of a cohort of children born in Pelotas, Brazil, was studied at 12 months of age regarding their development. The Denver II Test was used. The children who failed in two or more items of the test were suspected of having development delay. A set of independent variables was chosen taking into account the hierarchical relations between risk factors according to the conceptual framework (socioeconomic, reproductive and environmental, birth conditions, childreńs care, nutrition and morbidity). Analyses were performed using Mantel-Haenszel X2 and multivariate technique through conditional logistic regression, to control for possible confounding. RESULTS: At 12 months of age, 34% (463) of the total of 1,363 children failed in the screening test. After adjusting for possible confounding variables, failure was associated with family lower income children (OR= 1.5), very low birth weight (OR= 4.0), gestational age less than 37 weeks (OR= 1.6), more than three siblings (OR= 1.9), and duration of breastfeeding less than three months (OR=1.6), or no breastfeeding (OR= 1.9). Children who presented weight/age at six months of age less or equal to -2 z score of the reference population presented a risk 10 times greater of having failure in the Denver II Test. CONCLUSIONS: This study reinforces the multiple etiology of development delays and the concept of cumulative risk effect. In this population those who are economically disadvantaged accumulate risk factors (social, economic and environmental) that may render to deficits in their development.