Immune-mediated myositis and other forms of inflammatory myopathy are well recognised in dogs and man (Griffiths 1991). In equine medicine, inflammatory muscle disease is less well documented. Only a single report could be found in the literature containing mention of immune-mediated myositis in a horse unassociated with systemic disease, in which case the condition was multifocal (Beech 2000). We report a case of localised muscle atrophy and weakness in a pony. A diagnosis of chronic myositis was made after histological examination of biopsies of affected muscles. The histological appearance, elimination of other causes and response to treatment suggested an immune-mediated aetiology.