Parent- and teacher-report of attention-deficit/hyperactivity disorder (ADHD) symptoms were examined using problem behavior and DSM-IV symptom inventory questionnaires for 63 children with full mutation fragile X syndrome (FXS) and 56 children without disabilities matched on mental age (MA). Prevalence rates of ADHD symptoms varied depending on type of measure (problem behavior or DSM-IV criteria), subscale (ADHD-inattentive or ADHD-hyperactive), scoring method (continuous T-scores or categorical scores based on DSM-IV algorithm), and rater (parent or teacher). Overall, 54-59% of boys with FXS met diagnostic behavioral criteria for either ADHD-inattentive type only, ADHD-hyperactive type only, or ADHD-combined type based on parent or teacher report. Boys with FXS were rated as having clinically high scores or met diagnostic criteria at higher rates than expected for the general population and had higher raw scores than their MA-matched peers. Parent ratings of boys with FXS resulted in higher ADHD-inattentive type and ADHD-hyperactive type T-scores than teachers. Boys who were rated as meeting DSM-IV criteria were more likely to be taking psychotropic medication and to have younger mental ages. Parents were substantially more likely than teachers to rate boys as meeting DSM-IV criteria for ADHD-inattentive type, while teachers were only slightly more likely than parents to rate boys as meeting DSM-IV criteria for ADHD-hyperactive type. Teachers were more likely than parents to rate boys as meeting DSM-IV criteria for ADHD when boys had lower levels of FMRP.