Reasons for performing study: A neurological disorder characterised by pelvic limb metatarsophalangeal joint extensor paresis has been observed in numerous horses in Scandinavia for the last decade. Very little has been formally reported and there have been no detailed assessments of the neurological signs or neuropathological lesions. Objectives: To describe the epidemiological and pathological features of an outbreak of 'Scandinavian knuckling syndrome' in a riding stable in southern Finland. Methods: Clinical neurological examination of 4 cases and neuropathological assessment of tissues of one case were performed. Results: Eleven out of 17 horses fed on ryegrass from a common source showed progressive clinical signs of metatarsophallangeal extensor paresis necessitating euthanasia of 7 horses. Nervous system lesions in one horse consisted of a novel demyelinating, mildly inflammatory peripheral neuropathy, with BiP/GRP positive rough endoplasmatic reticulum Schwann cell inclusions. Conclusions: The clinical signs and lesions documented differ from any previously described equine polyneuropathy and suggest a primary Schwann cell lesion. Potential relevance: The classification of this disease as a novel demyelinating polyneuropathy may assist focused epidemiological investigations.