We describe a series of 13 patients, ten males and three females, with ages ranging between three and fourteen years, who show unilateral polymicrogyria (in the left cerebral hemisphere in four cases and in the right in nine). The first and the main clinical alteration in all 13 cases was hemiparesis that did not change along the years. Ten patients had seizures that were completely controlled in seven. Three cases never presented seizures. Spontaneous threat of abortion or arterial hypertension during the first five months of pregnancy occurred in five cases. Weight less than 3 kg at birth was observed in six cases. The only patient who shows severe mental retardation associated with deafness is a girl who had infantile spasms at 20 days of age and presents duplication of a short arm of the chromosome X. Conventional magnetic resonance (MR) studies, performed in four cases did not disclose polymicrogyria but only an enlarged cortex that was diagnosed as cortical dysplasia. Three-dimensional MR (3DMR) images are very important not only to see the polymicrogyria, but also its extension and severity, especially if, moreover the axial, coronal and sagittal views, oblique, frontal, occipital, basal and superior images of the hemispheric cortical surface are performed.