Aims: To examine the associations between being overweight and behavioural pro blems at ages 5 and 14. Methods: Birth cohort study of 2875 individuals who were born in Brisbane between 1981 and 1984 and who were followed up at ages 5 and 1 4 years. Behavioural problems were defined as scoring above the 90th centile on Achenbach’s child behavioural checklist. Results: In cross-sectional analyses there was no association between being overweight and behavioural problems in ei ther females at age 5. At age 14 females who were overweight were more likely th an those who were normal weight to experience behavioural problems. However, the re was no association between being overweight and behavioural problems at age 1 4 among males. The prevalence of behavioural problems increased linearly across the di stribution of body mass index in females at age 14. In prospective analyses, amo ng participants who had no behavioural problems at age 5, there was no associati on between being overweight at age 5 and behavioural problems at age 14 in eithe r sex. Females who were overweight at age 5 and normal weight at age 14 had redu ced odds of behavioural problems at age 14. Conclusions: Among adolescent female s there is a positive linear association between body size and behavioural probl ems. However, no such association was found in adolescent males, or in either se x at age 5 years, and in prospective analyses being overweight at age 5 was not associated with behavioural problems in either sex at age 14. Aims: To examine the associations between being overweight and behavioral pro blems at ages 5 and 14. Methods: Birth cohort study of 2875 individuals who were born in Brisbane between 1981 and 1984 and who were followed up ages 5 and 1 4 years. Behavioral problems were defined as scoring above the 90th centile on Achenbach’s child behavioural checklist. Results: In cross-sectional analyzes there was no association between being overweight and behavioral problems in ei ther females at age 5. At age 14 females who were overweight were more likely However, the re was no association between being overweight and behavioral problems at age 1 4 among males. The prevalence of behavioral problems increased linearly across the di stribution of body mass index in females at age 14. In prospective analyzes, amo ng participants who had no behavioral problems at age 5, there was no associati on between being overweight a t age 5 and behavioral problems at age 14 in eithe r sex. Females who were overweight at age 5 and normal weight at age 14 had redu ced odds of behavioral problems at age 14. Conclusions: Among adolescent female s there is a positive linear association between body size and behavioral probl ems. However, no such association was found in adolescent males, or in either se x at age 5 years, and in prospective analyzes being overweight at age 5 was not associated with behavioral problems in either sex at age 14 .