Castleman's lymphoma is a rare clinicopathological entity and is often difficult to classify nosographically. From the histopathological point of view two variants are recognized, one hyalino-vascular and one plasma cellular. Moreover Castleman's lymphoma may be multifocal or unifocal (located within the thorax in 70% of patients). The plasma cellular variant is often associated with systemic symptoms such as fever, sweating, organomegaly, polyneuropathy, and mono- or polyclonal gammopathy. We describe a rare case of unifocal, plasma cellular-type Castleman's lymphoma located in the mediastinum, asymptomatic at onset but later dominated by neurological symptoms (chronic isolated polyneuropathy whose clinical expression was predominantly motor). A brief review is presented of the main clinico-histological characteristics of Castleman's lymphoma together with some hypotheses about the pathogenesis of the associated neuropathy.