Recent studies have suggested an association between birth order and psychopathology. However, the association remains unclear due to a lack of consideration of the confounding effects of family size. At age seven years the family size and birth order of children from a large cohort of New Zealand children were examined in relation to parental help seeking for behavioural and emotional problems, and the development of DSM-III (American Psychiatric Association, 1980) disorder in pre-adolescence and adolescence. Neither the child's gender, birth order nor family size were associated with parental help seeking or the prevalence of DSM-III disorder in pre-adolescence at age 11. In adolescence at age 15, however, significantly more girls, first-borns and children from small families had DSM-III disorder, but the interactions between these factors were not significant. In particular, these associations were found with internalizing disorders. The appropriateness of further studies of birth order in relation to psychopathology is discussed.