Russell A. Parker, BVSc , Bruce M. Bladon, BVM&S, DESTS, Diplomate ECVS , Kate McGovern, BvetMed , and Ken C. Smith, BVM&S, FRCPath, FRCVS Veterinary Surgery July 2010 39 5 644 - 648 Objective: To describe and discuss previously unreported complications associated with intraosseous perfusion with gentamicin in horses. Study Design: Case report. Animals: Ten-year-old Warmblood gelding. Methods: Intraosseous perfusion with gentamicin into the proximal phalanx (P1) was used as part of the treatment regimen for distal interphalangeal joint and navicular bursa synovial sepsis. Although the sepsis responded favorably complications developed at the perfusion site, including persistent osteomyelitis, progressive osteonecrosis, and ultimately pathologic fracture of P1. Results: The progression of the clinical signs and findings at necropsy are suggestive of a toxic osteonecrosis secondary to intraosseous perfusion. Conclusions: Further work is needed to investigate the effects of high dose gentamicin on equine mesenchymal cells that may be achieved during intraosseous perfusion. Clinical Relevance: Lower doses of perfusate within the medullary canal of P1 or alternative perfusion sites should be considered.