This study is aimed to describe the morpho-pathological and etiopathogenic aspects of a case of granulomatous myositis in a horse. A 12 years old Quarter horse mare was evaluated because of the presence of different subcutaneous nodules and masses. The lesions were cytologically diagnosed as pyogranulomas and were resistant to the given pharmacological treatment. The persistence of the lesions along with the deterioration of the clinical status suggested to surgically remove three of them. Macroscopically they were fixed to the muscles, painless and firm in consistency. On cut section, they showed a purulent exudate. Tissue samples from the lesions were fixed in 10% buffered formalin and paraffin wax embedded. Histological sections were stained with H&E, PAS, Masson trichrome, Grocott and Gram. Molecular biology assay (PCR) was performed too. Histological examination revealed, in all samples, a diffuse mixed inflammatory infiltrate characterised by polymorphonuclear granulocytes (neutrophils and mainly eosinophils), macrophages, lymphocytes, plasma cells, epithelioid and multinucleated giant cells invading the endomysium. Different sized round foci of caseous necrosis with calcification and areas of collagenolytic degeneration were present in all sections. PAS and Grocott stains didn’t show fungi whereas Gram stain revealed the presence of blue pleomorphic Gram + microrganisms free or inside macrophages. The described results indicated a severe diffuse granulomatous myositis. PCR analysis revealed the etiologic agent as Corynebacterium pseudotuberculosis. Corynebacterium pseudotuberculosis infection occurs world wide as caseous lymphadenitis in small ruminants and granulomatous infection in horse and cattle. The bacterium can survive for extended periods in the environment and in the soil. Disease transmission is thought to occur thorough a contaminated environment by direct contact between animals and insects such as house flies (Musca domestica), stable flies (Stomoxys calcitrans), and other arthropods serving as mechanical vectors (1). Clinically, the infection in horses most commonly causes external abscesses or “pigeon fever” (90% of cases), however, internal abscesses (8%) or ulcerative lymphangitis (1%) may also occur (2). In our case, none of the known pathogenetic pathways reported in the literature seems to be completely comparable being some aspects common to the so called pigeon fever and others to the chronic form of ulcerative lymphangitis. The presented case is, therefore, very rare and interesting for the pathological findings and, overall, from the epidemiological point of view.