Background: Turner syndrome (TS) results from the loss of part or all of one X chromosome in females. It can result in short stature, various dysmorphic findings, and difficulties with psychosocial adjustment. Girls with TS have previously been found to exhibit increased levels of hyperactivity and inattention. However, no studies have assessed whether individuals with TS meet strict (DSM-IV) criteria for attention-deficit/hyperactivity disorder (ADHD). Objective: We looked at the prevalence of ADHD in girls with TS and evaluated the contribution of imprinting on cognitive performance (IQ) and ADHD. Methods: We tested 50 girls with TS for ADHD, IQ, academic performance, and parental origin of the X chromosome. Results: We report an 18-fold increase in the prevalence of ADHD in girls with TS (24%) compared with girls in the general population (1.3%) (p < .01) and a 4.8 fold increase when compared with boys and girls in the general population (5%) (p < .05). In contrast to previous reports, our molecular studies in females with 45,X also showed no association between IQ scores and the parental origin of the intact X chromosome. Conclusions: We find an increased prevalence of ADHD in girls with TS but no evidence for imprinting effects for cognitive performance.