BackgroundThe increased proportion of UK children diagnosed with autism spectrum disorder (ASD) has been attributed to improved identification, rather than true increase in incidence.AimTo explore whether the proportion of children with diagnosis of ASD and/or the proportion with associated behavioural traits had increased over a 10-year period.MethodA cross-cohort comparison using regression to compare prevalence of diagnosis and behavioural traits over time. Participants were children aged 7 years assessed in 1998/1999 (n=8139) and 2007/2008 (n=13831).ResultsDuring 1998/1999, 1.09% (95% CI 0.86–1.37) of children were reported as having ASD diagnosis compared with 1.68% (95% CI 1.42–2.00) in 2007/2008: risk ratio (RR)=1.55 (95% CI 1.17–2.06), P=0.003. The proportion of children in the population with behavioural traits associated with ASD was also larger in the later cohort: RR=1.61 (95% CI 1.35–1.92), P<0.001. Increased odds of diagnosis at the later time point was partially accounted for by adjusting for the increased proportion of children with ASD-type traits.ConclusionsIncreased ASD diagnosis may partially reflect increase in rates of behaviour associated with ASD and/or greater parent/teacher recognition of associated behaviours.