Background: Despite an expanding epidemiological evidence base, uncertainties remain over key aspects of the epidemiology of the "antisocial" disorders in childhood and adolescence. Methods: We used cross-sectional data on a nationally representative sample of 10,438 5-15-year-olds drawn from the 1999 British Child Mental Health Survey to examine age trends, gender ratios and patterns of comorbidity in DSM-IV Conduct Disorder (CD) and Oppositional Defiant Disorder (ODD). Results: CD was significantly more common in boys than girls, and increased in prevalence with age. Among children who met diagnostic criteria for CD, status violations and other non-aggressive conduct problems increased with age, while aggressive symptoms became less common. Gender differences in ODD varied by reporter. Estimates of age trends in ODD depended heavily on treatment of overlaps with CD. Following DSM-IV guidelines (where ODD is not diagnosed in the presence of CD), rates of ODD fell with age; if that constraint was released, clinically significant rates of oppositionality persisted at similar levels from early childhood to middle adolescence. CD and ODD showed high levels of overlap, and both diagnoses showed substantial comorbidity with other non-antisocial disorders. Conclusions: Results from this large-scale study confirm and extend previous findings in the epidemiology of the disruptive behaviour disorders.