We correlated neuroanatomical developmental parameters with sequential ultrasonography scans to reveal the structural basis of functional recovery after early focal hypoxic lesions of the human frontal lobe in premature infants. We studied the transient fetal subplate zone in the premotor and prefrontal cortex in premature, newborn, infant, and young adult brains by acetylcholinesterase (AChE) histochemical, Golgi, and immunocytochemical methods. The structural in vivo rearrangements of the cerebral wall after perinatal lesions were studied on serial real-time sector scans (5-MHz transducer). The subplate zone contains "waiting" axons and randomly oriented fetal neurons, its developmental peak is between 22 and 34 weeks of gestation, and it is present in the frontal cortex of newborns and disappears after the sixth postnatal month, but individual subplate-like neurons remain until adulthood. Ultrasonography revealed remarkable structural rearrangements of the cerebral wall when the hypoxic lesion occurred during the developmental peak of the subplate zone: anechoic cavities ("cysts") develop rapidly (within 3 weeks) in premature brains, the rebuilding of these lesions continues after birth, and cavities disappear around the 11th month. We propose that the transient population of "waiting" axons and cells of the subplate zone participate in the structural and functional plasticity of the human cerebral cortex after perinatal brain damage.