Purpose : To demonstrate activation signals within the cerebellar cortex and to determine the side of the cerebellar cortex eliciting activation signals in response to complex motor tasks, as seen on EPI fMRI Materials and Methods : Seven right-handed subjects (M : F = 3 : 4 ; mean age, 30.3 years) underwent repetitive finger apposition with the dominant right hand. Using a 1.5 T MRI scanner, EPI fMR images were obtained. MR parameters used for EPI fMRI were TR/TE/Flip angle : 0.96 msec/64 msec/90$^{circ}$, FOV 22cm, 128 $ imes$ 128 matrix, 10 slices, 10 mm thickness while those for SE T1 weighted localized images were TR/TE : 450/16, FOV 23cm, 256 $ imes$ 256 matrix. The paradigm was three sets of alternate resting and moving fingers for six cycles, resulting in times of 360 seconds (10 slices $ imes$ 15 EPI $ imes$ 6 cycles = 900 images). Image processing involoved the use of a 200mHz Dual Pentium PC with homemade software. T-testing (p < 0.005 ~ p < 0.0005) and time series analysis were performed, and to verify the locations of activated regions, resulting images were analyzed in a color-coded overlay to reference T1-weighted spin echo coronal images. percentage change in signal intensity (PCSI) was calculated from the processed data. Results : All normal subjects showed significant activation signals in both the contralateral (left) primary motor cortex (PCSI = 3.12% $pm$ 0.96) and ipsilateral (right) cerebellar cortex (PCSI = 3.09%$pm$ 1.14). Signal activation was detected in the contralateral supplemental motor area (2.91% $pm$ 0.82), and motor activation in the anterior upper half of the contralateral cerebellum (PCSI = 2.50% $pm$ 0.69). The difference in activation signals between both sides of the cerebellar cortex was not statistically significant. All data were matched with time-series analysis. Conclusion : Bilateral cerebellar activation is associated with unilateral complex finger movements, as seen on fMRI. This result maysupport the recent neurological observation that the cerebellum may exert bilateral effects on motor performance.