This prospective, blinded study evaluates the effect of in utero cocaine exposure on outcome of nonasphyxiated, term and near-term children born to women of low socioeconomic status. Two hundred nineteen children (101 cocaine-exposed and 118 control) with extensive natal evaluations are evaluated at 6-month intervals. We report here growth, performance on the Bayley Scales of Infant Development (BSID) through 30 months of age, and tone and reflexes at 6 and 12 months. To date, subjects have had 816 follow-up visits, with subject retention greater than or equal to 73%. Cocaine-exposed children showed statistically lower mean weights and smaller mean head circumferences than control children over the 30-month follow-up period (p < or = .011). The percentage of children with abnormal tone and reflexes, however, was similar in the two groups at 6 and 12 months (p > or = .34). Mean BSID Mental Development Index (MDI) and Psychomotor Development Index scores did not differ between the two groups (p > or = .16), although both groups' scores decreased over time (p < .001). Of concern, both cocaine-exposed and control groups had lower mean MDI scores than those published for a group of children of higher socioeconomic status. We conclude that, in our cohort of children, low socioeconomic or minority status may have had a substantial influence on BSID scores whereas in utero drug exposure did not.