We have studied the temporal and spatial characteristics of the development of unipolar brush cells (UBCs) in the human cerebellar vermis. Consistently with previous studies in rodents and cat, we have found that unipolar brush cells appear at a relatively late phase of cerebellar development and their development continues up to and beyond the first postnatal year. A series of 23 normal human brains, including 5 adult and 18 fetal or infant brains (between the 24th gestational week and the 11th postnatal month) were used. In order to visualize unipolar brush cells, calretinin-immunocytochemistry was performed on formaldehyde-fixed, paraffin-embedded blocks of the cerebellar vermis. Our results show that calretinin-immunoreactive unipolar brush cells are not yet present in the cerebellar vermis at the 28th gestational week. At birth, they are present in a relatively small number, mostly in the vestibular lobules. At the 3rd, 5th, 8.5th and 11th postnatal months the number of calretinin-immunoreactive unipolar brush cells gradually increase, first appearing in the vestibular lobules, followed by the invasion of the later developing vermal lobules, spreading in a rostro-caudal and proximo-distal direction. Although at the 11th postnatal month unipolar brush cells exhibited adult-like morphological and distributional features, their number appeared to be lower than in the adult cerebellum. The late maturation of unipolar brush cells implies that the cytoarchitectonical development of the human cerebellum is not completed by the end of the first postnatal year.