At all ages XXX girls had significantly smaller head circumferences than control girls. Their IQ deficit was 24 points and IQ at age seven correlated significantly with head circumference at birth. In XXY boys, head circumference was significantly reduced at birth and up to nine years of age. The XXY boys' IQ deficit was 22 points, but IQ did not correlate with head circumference, as reductions in the two parameters did not always occur in tandem. The ratio of height-to-head circumference differed most in this group and could be useful in clinical recognition of this condition. XYY boys' head size did not differ from controls, despite their greater height, lower IQ scores indicating an adverse effect of an additional Y chromosome on brain development. The major factor affecting IQ outcome in the cohort was abnormal karyotype, with smaller effects from social class and head growth.