Four patients with polyneuropathy complicating the plasma cell variant of Castleman's disease (angiofollicular lymph node hyperplasia) are described. The neuropathy was predominantly motor and severely disabling. Vasculopathy, papilloedema, organomegaly, endocrinopathy, oedema and paraproteinaemia were variably present in these patients. Sural nerve biopsy showed changes of both demyelination and axonal loss. Capillary proliferation and endothelial hypertrophy in the epineurium and endoneurium, similar to that seen in affected lymph nodes, suggested that a diffuse vasculopathy may contribute to the neuropathy. Serum antibody activity against a variety of neural antigen preparations was not detected in any of the patients. Two untreated patients died. Substantial improvement in the neuropathy occurred in the two patients treated with cyclophosphamide and prednisolone.