Magnetic resonance imaging (MRI) studies of the caudate nucleus have reported reversal asymmetry and alterations of its size, suggesting a striate cortical disorder related to attention-deficit hyperactivity disorder (ADHD). The objective was to evaluate whether alterations of the asymmetry and size of the caudate nucleus head exist in a sample of well-controlled 7- to 11-year-old Colombian children, with different types of ADHD. Two groups of cases—ADHD of the combined type and ADHD of the inattentive type—and one control group, were selected. Multiple methods for assessing ADHD (rating scales, psychologic interview, neurologic history and examination, and neuropsychologic evaluation) were used to confirm the diagnoses. Participants with a history of language disorder, learning disabilities, depression, and other major neurologic and psychiatric conditions were excluded. Finally all groups had 15 children, matched by sex (7 male, 8 female), age, socioeconomic status, and grade. Height, weight, head circumference, and encephalic index were statistically controlled. Three T 1 -weighted volumetric (three-dimensional) MRI slides of the caudate nucleus head were obtained with a 1.5-Tesla Gyroscan apparatus. The control group had a significantly higher Wechsler Full-Scale IQ than the groups with ADHD of the combined type and ADHD of the inattentive type (P < .001). Volumes from the left caudate nucleus head were significantly larger than volumes from the right in all groups (P < .001). There were no group differences when volumes were compared between groups. All of the groups had left caudate nucleus head volumes significantly higher than right, although there were no between-group differences. The results in relation to previous studies are discussed. (J Child Neurol 2002;17:97-105).