A 9-month-old male neutered mixed-breed cat had a history of chronic lameness of the right hind limb, which was non-responsive to antibiotic treatment. Hematologic analysis revealed marked neutrophilia and mild monocytosis. Radiography revealed extensive loss of cortical bone, and replacement with irregular and disorganized bone. There was loss of the normal cortico-medullary distinction, and the medullary cavity had an irregular radiodensity suggestive of osteomyelitis. Surgical curettage and antibiotics did not improve the clinical condition, and amputation was performed. Grossly, the skin over the right tibia was ulcerated with a viscous and granular exudate. At histopathology, there was marked diffuse pyogranulomatous dermatitis, myositis, periostitis and osteomyelitis associated with Splendore-Hoeppli phenomenon. In addition, there was marrow osteoproliferation and multifocal cortical loss, reabsorption, fibroplasia and endosteal bone formation. Gram staining revealed myriad slightly elongated Gram-positive bacteria, arranged in pairs or single chains, confirmed by polymerase chain reaction as Streptococcus species.