PURPOSE: The purpose of this study was to evaluate the MR findings and useful sequences inPolymyositis/Dermatomyositis, and to correlate MR findings with disease activity. MATERIALS AND METHODS: The studyincluded nine clinically proven cases of Polymyositis/Dermatomyositis, eight involving the thigh and one, theshoulder (2 cases, 1 follow-up). The contrast between affected and normal muscles and difference in signalintensity ratio in the muscle groups were retrospectively evaluated on Gd-enhanced T1WI and T2WI. We alsoevaluated the magnitude of involvement of muscle groups, fatty replacement of muscle and change of subcutaneousfat layer, and correlated signal intensity ratio with serum level of muscle enzymes. Differences in signalintensity ratio and the frequency of chemical shift artifact were evaluated on T2WI as active and inactive groupsclassified according to clinical findings, and the chemicalshift artifact was correlated with the finding ofGd-enhanced T1WI. Exvept in the case of one shoulder, statistical analysis was assessed by the Anova test andt-test. RESULT: On Gd-enhanced T1WI and T2WI contrast was 0.54and 0.82, respectively and p value was 0.02. Withregard to difference in signal intensity ratios of muscle groups, as seen on Gd-enhanced T1WI and T2WI, p valveswere 0.07 and < 0.01, respectively. Muscle involvement was thus clearly visualized on T2WI. The order of frequencyof involved muscle groups was vastus muscles, gluteus maximus, sartorius muscles, adductor muscles, gracilismuscle, and hamstring muscles. Fatty replacement and subcutaneous fatty change were visualized in five cases andone, respectively. The correlation coefficient between the signal intensity seen on T2WI and muscle enzymes was0.59 (CPK) and 0.52 (LDH). The chemical-shift artifact was detected in both clinical groups (four active twoinactive) and corresponded to one case of muscle involvement and five of perimuscular edema, as seen onGd-enhanced T1WI. CONCLUSION: T2WI is useful for the evaluation of muscle involvement and correlated closely withdisease activity; signal intensity ratio could not be substituted for the serum level of muscle enzymes. The groupof thigh muscles most affected was the vastus muscles, while the hamstring muscles were least affected. Thechemical-shift artifact corresponded mainly to perimuscular edema and did not correlate with disease activity.