Primary ovarian lymphoma is rare. To better understand the clinicopathological and molecular features of these tumours, we studied 14 patients (age range 13 to 74 years, mean 46.9 years) with non-Hodgkin's lymphoma who initially presented with an ovarian tumour. The clinicopathological features were reviewed and tissue samples were studied by immunohistochemistry and for immunoglobulin gene rearrangement, presence of Epstein Barr virus and for MYC and BCL-2 rearrangements by fluorescence in situ hybridisation. The most common symptoms were abdominal pain and abdominal distension. Ten patients had a unilateral (7 left, 3 right) ovarian mass and 4 had bilateral ovarian masses, size range 4 to 13 cm (mean, 7.9 cm). Of the 14 lymphomas, 12 were classified as diffuse large B-cell lymphoma (DLBCL) and 2 as Burkitt lymphoma. The Burkitt lymphomas had a MYC rearrangement. The DLBCLs were of the germinal centre B-cell-like subtype. One DLBCL patient had MYC and BCL2 rearrangements. The treatment consisted of various combinations of surgery, chemotherapy and radiotherapy. The follow-up period ranged from 3 to 56 months. During the follow-up period, 8 patients died of disease, 4 were alive with disease, and 2 were alive without disease. Most primary ovarian lymphomas are of germinal centre B-cell-like subtype DLBCL, which have high proliferative activity and poor prognosis.