A series of 100 consecutive orbitotomies for tumour was assessed. Lymphoid infiltrations contributed 27% and malignant non-Hodgkin lymphoma was the commonest primary malignant orbital tumour in this series. The clinical and histological features of the first histologically proven case of primary orbital Castleman's disease are reported, as are the results of a study of 27 patients with pure orbital lymphoid infiltrations. The natural history of conjunctival lymphomas differs significantly from that of pure orbital lymphomas and for this reason no conjunctival lymphomas were considered in this series (which therefore contrasts with some previous reports). Histological examination with fresh unfixed tissue available for immunohistochemical staining enhances the diagnostic accuracy of these varied underlying diseases. More than one biopsy may be required before the diagnosis is established. These are significant factors given the differing lines of management for each disorder.