Abnormalities of the cerebral cortical pattern have been observed in association with the Chiari Type II malformation and described as polymicrogyria, microgyria, stenogyria or polygyria. In order to characterize and quantify these abnormalities, the sulcal patterns of the brains of 15 patients with the Chiari Type II malformation were compared with those of 10 pediatric controls without cerebral malformations. In this analysis, sulci were grouped by anatomical characteristics into three categories. The first category, the fissures, were represented by the long, deep indentions in the cerebral cortex such as the Sylvian fissure. The second category, the principal sulci, were represented by the shorter, named, anatomically consistent sulci such as the superior frontal sulcus. The third category contained the minor sulci which were defined as those shallow grooves which depressed the cortical surface but were not recognizable as consistent anatomical landmarks. Subjectively all brains with Chiari Type II malformations demonstrated abnormal gyral patterns. These brains exhibited abnormalities in the number and distribution of principal and/or minor sulci. Sulcal density, defined as the length of sulcus per square millimeter of cortical area, was quantitated utilizing a computer assisted digitizer program. For the brains with the Chiari Type II malformations, the length of all sulci (principal and minor combined) per square mm was 0.238 ± 0.045 mm -1 as compared with the control patients whose value was 0.182 ± 0.021 mm -1 (t = .0015). Average length per square millimeter of principal sulci was 0.176 ± 0.029 mm -1 as compared with 0.132 ± 0.015 mm -1 (t = .0002) for the control brains. Although the minor sulcal length per square millimeter of all the brains with the Chiari malformation were not statistically greater than controls (t = 0.17), a subset of 6 brains with the Chiari malformation were identified in which this measurement was significantly greater than control values at the 0.99 level. The study makes clear that cerebral gyral abnormalities are a prominent feature in the Chiari Type II malformation and that the presence of polygyria in this myelodysplastic syndrome does not necessarily prevent these patients from developing intellectual abilities in the normal range.