Mothers of 60 girls with fragile X [fra(X)] and 58 controls were asked to fill out the Conners' Parent's Questionnaire to determine if this commonly used checklist would detect behavior problems in young girls with fra(X). The mean T-scores were significantly different between the 2 groups of girls on 5 of the 6 factors on the Conners' Parent's Questionnaire and the girls with fra(X) had mean T-scores that were above the average range on both the Anxiety factor and the Hyperactivity Index factor. Significantly more girls had T-scores greater than 70 (> 98th centile) on 3 of the 6 factors. Twenty-three percent of the girls had T-scores greater than 70 on the Anxiety factor and 38% of the girls had T-scores greater than 70 on the Hyperactivity Index factor. This study suggests that some behavior problems will be reported frequently in young girls with fra(X) and should not be difficult for clinicians to detect.