Background: The objective of the present study was to describe the development of boys and girls during the elementary-school years on three dimensions that conceptually and empirically represent risk for maladjustment. Method: Every year between kindergarten and grade six, teachers rated the impulsivity, fearfulness, and helpfulness dimensions among a sample of 1,865 children representative of kindergarten boys and girls in the province of Québec (Canada) in 1986–87. A group-based trajectory method was used to 1) identify groups of boys and girls following distinct-level trajectories of behaviours (on each dimension) during the elementary-school years; 2) estimate the proportion of children in each of the identified trajectory groups; and 3) estimate the patterns of consistency and variations in trajectories. Results: The results indicated that the best models comprised three distinct-level trajectory groups on fearfulness and helpfulness (a low, moderate, and high group) and four distinct-level trajectory groups on impulsivity. The helpfulness and fearfulness trajectory groups were generally more stable than the impulsivity groups. The broad patterns of development were similar across sexes. However, there were more boys on the higher impulsivity trajectories and low helpfulness trajectory, while there were more girls on the high fearfulness trajectory. Conclusion: We found that behavioural consistency over middle childhood varied across trajectory groups and across dimensions, and we identified sex differences in the distribution of children in the different trajectory groups that may reflect gender-specific risks for psychopathology.