Biopsy of a middle ear mass can lead to facial paralysis when the mass is a facial nerve schwannoma. Immunostaining techniques were used to determine if nerve fibres might be present within the tumour mass. Nerve fibres were found to be widely disseminated throughout the tumour in eight of 23 facial nerve schwannomas. Antibodies to neurofilaments demonstrated single fibres, or small groups of fibres, that are very difficult to see with routine haematoxylin and eosin stains. The presence of fibres throughout some tumours makes biopsy of them likely to result in facial dysfunction.