Whether preschool males with fragile X syndrome can be distinguished from those with idiopathic developmental delay in the four problem behavior areas associated with the fragile X phenotypes was examined. Males with fragile X (n = 41) and age- and IQ-matched controls (n = 16) were rated by their mothers on the Dimensions of Temperament Scale-Revised, the Child Behavior Checklist, and the Aberrant Behavior Checklist--Community. The fragile X group showed deficits in motor skills, increased initial avoidance, decreased social withdrawal, deficits in attention, increased hyperactivity, and positive mood. They were distinguished from controls on all of these variables except hyperactivity and attention. When maternal characteristics were controlled for, the fragile X group showed a significantly higher level of generalized activity level than did controls.