OBJECTIVES: To investigate the developmental impact of a prenatal-to-age-5 multicomponent early intervention program targeting families living in low socioeconomic conditions. METHODS: Pregnant women from a disadvantaged Irish community were randomly assigned into a treatment group (home visits, baby massage, and parenting program; n = 115) or control group (n = 118). Children's behavioral problems (externalizing, internalizing), cognitive skills (general, vocabulary), and health service use (number of health clinic visits), were regularly assessed (6 months to 4 years of age). Children's developmental trajectories were modeled by using latent class growth analyses to test whether certain subgroups benefited more than others. RESULTS: High and low developmental trajectories were identified for each outcome. Treated children were more likely to follow the high-level trajectory for cognition (odds ratio = 2.89; 95% confidence interval = 1.55-5.50) and vocabulary skills (odds ratio = 2.02; 95% confidence interval = 1.08-3.82). There were no differences by treatment condition in the risk of belonging to a high externalizing or high health clinic visit trajectory. However, within the high externalizing trajectory, treated children had lower scores than controls (Hedges' g range (2-4 years) = 0.45-0.58; P < .05) and, within the high health clinic visit trajectory, only children in the control group experienced an increasing number of visits. CONCLUSIONS: This program revealed moderate positive impacts on trajectories of cognitive development and number of health clinic visits for all children, whereas positive impacts on externalizing behavior problems were restricted to children with the most severe problems.< Réduire