An 18-month-old 450-kg [990-lb] sexually intact male Holsteiner was evaluated for lameness of the left forelimb of 3-months' duration. We were unable to localize the site of lameness, using intra-articular and perineural anesthesia, and radiography of the shoulder and cubital joint (elbow) did not reveal radiographic abnormalities. Nuclear scintigraphy was performed. An increase in radio-isotope uptake was evident at the lateral epicondyle of the left humerus. Radiographs of the region 3 weeks later revealed a 1.5-cm focal lucency surrounded by a 1.0-cm rim of necrotic bone. The lesion was consistent with an encapsulated osteomyelitis or bone abscess. Surgical correction was performed, using general anesthesia, and involved a lateral approach to the epicondyle of he humerus. Cyst contents were removed with curettage, and the cavity was packed with cancellous bone harvested from the tuber coxae. A coagulase-negative Staphylococcus organism was cultured from the abscess. The horse was sound 3 weeks after surgery, and radiography 10 months later revealed complete ossification and adjacent sclerosis at the surgical site. Solitary cystic lesions of long bones may represent bone abscesses capable of causing lameness in horses. Nuclear scintigraphy can provide early diagnostic capability. Curettage and cancellous bone grafting are indicated for treatment of bone abscesses.