How many cerebellar granule cells are generated pre- or postnatally in human is unknown. Using a rigorous design-based stereologic approach we investigated postmortem cerebella from 14 children who died between the first postnatal day (P1) and 11 months of age (M11). We found a statistically significant (p < 0.05) age-related increase in the total number of granule cells from 5.9 × 10(9) at M1 to 37.6 × 10(9) at M10/11 per cerebellar half but not in the total number of Purkinje cells (12.1 × 10(6) at M1 vs. 13.9 × 10(6) at M10/11 per cerebellar half). Accordingly, approximately 85 % of the cerebellar granule cells are generated postnatally in human, and the number of granule cells per Purkinje cell in the human cerebellum increases from 485 at M1 to 2,700 at M10/11, approximately. These data indicate that the human cerebellum has a much higher functional plasticity during the first year of life than previously thought, and may respond very sensitively to internal and external influences during this time. This has important implications for several neuropsychiatric conditions in which cerebellar involvement has been demonstrated.