Aim To investigate psychopathology in children with neurofibromatosis type 1 (NF1), particularly the prevalence of autism spectrum disorder (ASD) and attention-deficit-hyperactivity disorder (ADHD) symptomatology, using a population-based sampling approach. Method Standard questionnaire screen reports were analysed for ASD (Social Responsiveness Scale, SRS), ADHD (Conners' Parent Rating Scale- Revised, CPRS-R), and other psychiatric morbidity (Strengths and Difficulties Questionnaire, SDQ) from parents and teachers of children aged from 4 to 16years (112 females, 95 males) on the UK North West Regional Genetic Service register for NF1. Results Parental response rate was 52.7% (109/207 children; 59 females, 50 males, mean age 9y 11mo, SD 3y 3mo). The SRS showed that in 29.4% (32/109) of children, autism was in the severe, clinical range (T-score>75) and in 26.6% (29/109) in the mild to moderate range (T-score 60-75). CPRS-R scores showed that in 53.8% (57/106) of children autism was in the clinical ADHD range (ADHD index T-score>65). Based on their scores on the SDQ total difficulties scale, 41.5% (44/106) of children were in the abnormal range and 14.2% (15/106) were in the borderline range. Twenty-five per cent (26/104) of children met criteria for both clinical autism and ADHD. Interpretation This representative population-based sample of children with NF1 indicates a high prevalence of ASD symptoms associated with NF1 as well as substantial co-occurrence with ADHD symptoms. The findings clarify the psychopathology of NF1 and show the disorder as a potentially important single-gene cause for autism symptoms. © 2012 Mac Keith Press.