AS CHILDREN, ALL OF THESE MEN WERE SLOW LEARNERS. THEY MANIFESTED DELAYED SPEECH DIFFICULTIES, CLUMSINESS, MILD TREMORS, LACK OF MUSCULAR CONTROL, CONVULSIONS, EITHER ABNORMALLY SHY OR HYPERACTIVE BEHAVIOR, DISTRACTIBILITY, IMPULSIVENESS, AND WEAK MENTAL CONTROL. ALL HAD PSYCHOSEXUAL DISTURBANCES AND HAD COMMITTED CRIMES. PNEUMOENCEPHALOGRAMS SHOWED GENERAL VENTRICULAR ENLARGEMENT. THE LATERAL VENTRICLES WERE EITHER UNILATERALLY OR ASYMMETRICALLY BILATERALLY ENLARGED, WHILE THE SUPRAPINEAL RECESSES OF THE THIRD AND FOURTH VENTRICLES WERE UNIFORMLY ENLARGED. THE CEREBELLUM WAS ALSO ABNORMAL. FOUR OF THE MEN WERE SIGNIFICANTLY TALLER THAN THE GENERAL FINNISH MALE POPULATION. THE FINDINGS ARE SIMILAR TO THOSE SEEN IN CHILDREN WITH SO-CALLED MINIMAL BRAIN DYSFUNCTION (MBD) SYNDROME. THE XYY SYNDROME MAY BE ONE REASON WHY MOST MBD SUFFERERS ARE MALES AND WHY SUCH ADULT PERSONALITY DISORDERS AS CRIMINAL PSYCHOPATHY ARE FOUND PREDOMINANTLY AMONG MALES. CHROMOSOME ANALYSIS IS IMPORTANT NOT ONLY FOR ADULT PSYCHOPATHIC CRIMINALS BUT ALSO FOR MBD BOYS AND MALES WITH PSYCHOSEXUAL OR OTHER PERSONAL IMMATURITY, ESPECIALLY IF THEY ARE TALL. PHOTOGRAPHS ILLUSTRATE THE CHANGES FOUND IN THE BRAIN. TABLES COMPARE THE CLINICAL FINDINGS. REFERENCES ARE APPENDED. ABI GLR