We report the case of a 27-week premature infant in whom magnetic resonance imaging (MRI) at 4 postnatal weeks (postconceptional age, 31 weeks), term, and 6 months of age documented the postnatal postmigrational evolution of bilateral perisylvian polymicrogyria. The polymicrogyria was readily detected by ultrafine 1.5-mm coronal slices on three-dimensional, Fourier-transformed, spoiled gradient-recalled and T2-weighted MRI sequences. These MRI sequences provide the first in vivo documentation of the postmigrational evolution of polymicrogyria. The likelihood that the polymicrogyria was related to an ischemic encephaloclastic mechanism is supported by the simultaneous occurrence of periventricular leukomalacia.