While ovarian involvement as a late manifestation of disseminated non-Hodgkin lymphoma (NHL) is not uncommon, primary ovarian NHL, or NHL presenting for the first time as an ovarian mass, is rare, having a reported incidence of around 0.5% of all untreated NHL and 1.5% of untreated ovarian neoplasms. This case study reports a 27-year-old woman who underwent a unilateral salpingo-ophorectomy and staging for a very large, rapidly enlarging, right ovarian mass, suspected clinically of being a dysgerminoma. On frozen section, the tumour was favoured to be a germ cell tumour, with juvenile granulosa cell tumour also not excluded. On permanent section, however, the tumour more clearly demonstrated morphological features and immunological features in keeping with a large B-cell non-Hodgkin lymphoma. Coupled with FISH analysis showing no evidence of a MYC gene rearrangement, a diagnosis of DLBCL, NOS, double expressor phenotype, was ultimately made. This case serves as a reminder that, although rare as an initial presentation in the ovary, lymphoma should be considered by pathologists in the differential diagnosis of ovarian tumours, including at frozen section, where consideration should also be given to obtaining fresh tissue for flow cytometry. 1.Dimopoulos MA, Daliani D, Pugh W, et al. Primary ovarian non-Hodgkin's lymphoma: outcome after treatment with combination chemotherapy. Gynecol Oncol 1997; 64: 446–50.