Stereological estimates of the numbers of Purkinje cell nucleoli in human cerebellar cortex have been obtained from systematic random samples of tissue by using the fractionator. The estimates are unbiased by fixation, section thickness, or sampling errors and are independent of any assumptions about cell shape, size, or spatial orientation. Twelve brains from aged subjects of both sexes were examined. The average complement of nucleoli in four female brains (age range 71-93 years) amounted to 14.8 millions (with an observed coefficient of variation between subjects of 29%). For three male brains (76-91 years), the corresponding estimates were 15.7 millions (10%). No significant sex differences were found for these small samples. Five brains of unknown sex and age yielded values of 15.8 millions (18%). For the twelve brains examined, the total number of Purkinje cell nucleoli per cerebellum was found to be 15.4 millions (19%). Estimated numbers showed a significant positive correlation with cerebellar weights. The number of nucleoli in an individual cerebellum was obtained with high precision in as short a time as 4 hours.