The UK prevalence of parent-reported autism spectrum disorder (ASD) and attention deficit/hyperactivity disorder (ADHD) were estimated from the Millennium Cohort Study. Case definition was if a doctor or health care professional had ever told parents that their child had ASD and/or ADHD. Data were collected in 2008/2009 for 14,043 children. 1.7% of children were reported as having ASD (95% CI 1.4-2.0) at mean age 7.2 years (SD = 0.2; range = 6.3-8.2). 1.4% reportedly had ADHD (95% CI 1.2-1.7), and 0.3% had both ASD and ADHD (95% CI 0.2-0.5). After adjusting for socio-economic disadvantage, only male sex (p < 0.001 for both conditions) and cognitive ability, p = 0.004 (ASD); p = 0.01 (ADHD) remained strongly associated. The observed prevalence of parent-reported ASD is high compared to earlier UK and US estimates. Parent-reported ADHD is low compared to US estimates using the same measure.