Though cross-sectional studies have yielded important information regarding the trajectory of psychopathology in middle childhood and adolescence in Down syndrome, there has been little exploration of maladaptive behaviour in the earliest years of development. In this study, we explore the emergence of maladaptive behaviour in young children with Down syndrome (n = 24) and a mental age-matched comparison group (n = 33) of young children with developmental disabilities of mixed etiologies. Behavioural data (Bayley Scales of Infant Development, Infant Temperament Questionnaire, Achenbach's Child Behaviour Checklist) were collected for children in each group at 12 months, 30 months, and 45 months. Findings from this study suggest that the onset of internalising behaviour difficulties in young children with Down syndrome may emerge later in early development than in children with developmental disabilities of mixed/nonspecific etiologies. In addition, temperament at Time 1 appeared to be a much stronger predictor of maladaptive behaviour outcomes at Time 3 in the mixed comparison group than in the Down syndrome group. Implications of this trajectory of the early development of maladaptive behaviour in Down syndrome for intervention for young children with Down syndrome are discussed. (Contains 3 tables.)