PURPOSE: To describe the findings of magnetic resonance imaging in infectious myositis and to determine theirvalue for differentiation between tuberculous and bacterial myositis. MATERIALS AND METHODS: Magnetic resonanceimages of ten proven cases of infectious myositis, (five tuberculous and five bacterial) were retrospectivelyreviewed in the light of clinical and laboratory findings. On the basis of magnetic resonance images, signalintensity of the mass, the presence or absence of an abscess, signal intensity of the peripheral wall, patterns ofcontrast enhancement, and associated findings were evaluated. RESULTS: Compared with those of bacterial myositis,the symptoms of tuberculous myositis lasted longer but there were no definite local inflammatory signs. In threeof five cases of bacterial myositis there were specific medical records ; trauma in two cases and systemic lupuserythematosus in one. All tuberculous myositis cases involved a single muscle, but bacterial myositis affectedmultiple muscles in three cases(60%). All but one case showed a mass in the involved muscles. In one bacterialcase, there was diffuse swelling in the involved muscle. On T1-weighted images, eight infectious cases showed lowsignal intensity ; two, of the bacterial type, showed subtle increased signal intensity. All cases demonstratedhigh signal intensity on T2-weighted images. The signal intensity of peripheral wall was slightly increased onT1-weighted images, but low on T2-weighted. In four cases there was associated cellulitis, and in one case each,adjacent joint effusion and deep vein thrombosis were seen. After gadolinium infusion, peripheral rim enhancementwas noted in nine cases and heterogeneous enhancement in one. CONCLUSION: After magnetic resonance imaging ofinfectious myositis, the characteristic finding was an abscessed lesion, with the peripheral wall showing highsignal intensity on T1-weighted images and low signal intensity on T2 weighted. Although we found it difficult todifferentiate bacterial from tuberculous myositis, magnetic resonance imaging findings and clinical manifestationsmay help in this respect.