Purpose : To evaluate the size of normal pons and cerebellum in vivo and the change in size according to age,and to compare those with measurement of the diseased pons and cerebellum. Materials and methods : 121 normaladults(M:F=54:67), 5 patients with OPCD and 19 patients with Wallerian degeneration were studied. The normal groupwas divided into 5 subgroups according to the age(ranged from 20 to 72 years). 1.5T GE Signa MR unit was used. Onaxial plane, the AP(A) and transverse(B) diameters of the pons, the size of the middle cerebellar peduncle(C), andtransverse diameter of the posterior fossa(D) and the cerebellum(E) were measured. On midsagittal plane, thelongitudinal(F) and AP(G) diameters of the basis pontis were measured. The ratios of E/D and F/G were calculated.The student t test was used for statistical analysis. Results : C, E and F/G were $15.5mmpm1.3,$ $99.8mmpm4.3$ $and1.63pm10,$ respectively. F/G, H/I, and H/J were larger in male(p<.01). All data of the pons showed no statisticallysignificant differences among age groups. E of the seventh decades was shorter than that of the thirddecades(p<.05). $C(12.7mmpm1.4)$ in OPCD and $F/G(1.81pm.10)$ in Wallerian degeneration(p<.01) showed the mostsignificant differences when they were compared to the normal. Conclusion : Although the cerebellum decreased insize with age, the pons maintained its size up to eighth decades. The measurement of middle cerebellar peduncle onaxial plane(C) and the ratio of basis pontis on midsagittal plane (F/G) were important in the evaluation of OPCDand Wallerian degeneration, respectively.