The inferior olive (IO) is the sole source of the climbing fibers that innervate the Purkinje cells of the cerebellar cortex. The IO comprises several subdivisions, the dorsal accessory olive (DAO), medial accessory olive (MAO), and principal nuclei of the IO (IOpr); the relative sizes of these subnuclei vary among species. In human, there is an expansion of the cerebellar hemispheres and a corresponding expansion of the IOpr. We have examined the structural and neurochemical organization of the human IOpr, using sections stained with cresyl violet (CV) or immunostained for the calcium-binding proteins calbindin (CB), calretinin (CR), and parvalbumin (PV), the synthetic enzyme for nitric oxide (nNOS), and nonphosphorylated neurofilament protein (NPNFP). We found qualitative differences in the folding patterns of the IOpr among individuals and between the two sides of the brainstem. Quantification of IOpr volumes and indices of folding complexity, however, did not reveal consistent left-right differences in either parameter. Single-label immunohistochemistry showed that populations of neurons in the IOpr express CB, CR, NPNFP, and nNOS. Individual fibers labeled for PV, CB, CR, NPNFP, and nNOS were also found. There was individual variability in the numbers and density of stained neurons in the human IOpr; such variability was not seen in other brainstem nuclei. These data are consistent with, and complement, earlier studies showing a dramatic age-related increase in lipofuscin and decrease in RNA in the human IOpr. The impact of these changes in the IOpr on cerebellar function is, however, not known.