Intermediate T2-weighted MR images and a systematic sampling stereological method were used in 37 normal volunteers, 24 to 79 years old, to assess the effects of age and sex on cerebellar volume. Female subjects (n = 21) had significantly smaller cerebellar volumes compared with males (n = 16) of similar age (t = -3.9, p less than .0008, two-tail t test). Using straight-line, univariate regression, we determined that age was not a significant predictor of cerebellar volume (R2 = 0.07, t = -1.66, p = 0.11), whereas gender did appear to account for a significant amount of variability in cerebellar volume (R2 = 0.33, t = -4.13, p = .0002). The mean absolute cerebellar volume in this study was 112 ml (SD +/- 16) for all subjects, 104 ml (SD +/- 10) for females, and 122 ml (SD +/- 16) for males. This study demonstrates the feasibility of using MR images along with a systematic stereological method to assess in vivo human cerebellar volume, thereby providing a research tool to correlate cerebellar morphology with cognitive and neuromotor function.