Background Few studies have explored the influence of preschoolers' behavioural problems on obesity treatment. Objectives To assess emotional and behavioural problems before and after an obesity intervention and examine relationships between changes in child behaviour and changes in weight status. Method The study included 77 children (4‐6 years old, 53% girls, mean body mass index [BMI] z‐score of 3.0 [ SD 0.6]) who participated in the More and Less Study, a randomized controlled trial. Families were randomized to a parenting program or to standard treatment. The children's heights and weights (BMI z‐score, primary outcome) were measured at baseline and 12 months post baseline. Parents rated their children's behaviours (secondary outcome) on the Child Behavior Checklist (CBCL) for ages 1.5 to 5 years, a questionnaire that measures psychosocial health and functioning, encompassing emotional and behavioural problems. Changes in child behaviour during treatment were examined through paired samples t tests; the influence of child behaviour on treatment effects was examined through linear regressions. Results Child emotional and behavioural problems significantly improved after obesity treatment. Lower scores were found for Emotional Reactivity, Sleep Problems, Affective Problems, Aggressive Behaviour, Externalizing Behaviours, Oppositional Defiant Problems, and Total Problems. Child behaviour significantly affected obesity treatment results: Attention Problems and attention deficit hyperactivity disorder (ADHD) at baseline contributed to increasing BMI z‐scores, whereas Oppositional Defiant Problems, Externalizing Behaviours, and a higher number of behavioural problems predicted decreasing BMI z‐scores. Conclusions Child behaviours at baseline influenced treatment results. Child emotional and behavioural problems improved post treatment. The results suggest that obesity treatment may help in reducing emotional distress among preschoolers.