A 7-year-old Paint mare presented for evaluation of a swollen right hindlimb of approximately 6 weeks' duration. Ultrasonography and radiography suggested a severe osteomyelitis and abscessation of the right tibia. Previous treatment included systemic antimicrobial therapy using procaine penicillin G (22,000 u/kg bwt, q. 12 h, i.m.) and gentamicin sulfate (6.6 mg/kg bwt, q. 24 h, i.v.) initially followed by oral doxycycline (10 mg/kg bwt, q. 12 h, per os) and rifampin (5 mg/kg bwt, q. 12 h, per os). Based on the poor prognosis the mare was subjected to euthanasia. Necropsy results subsequently revealed a severe, chronic, focal, necrotising osteomyelitis with localised cellulitis and abscessation. Rhodococcus equi was isolated from the tibia, subcutaneous abscess and from a pectoral abscess found at necropsy. Immunological testing of blood samples obtained ante mortem revealed normal blood lymphocyte phenotyping, but markedly increased IgG and IgM concentrations, indicating an active humoural response. While there have been reports of Rhodococcus equi infection in mature horses this is an area previously unreported in the literature.