Lymphoma should be considered early in patients with HIV when there is a history of weight loss. Although B-cell lymphoma is an AIDS-defining cancer, and many reports of lymphoma in HIV positive patients exist in the literature, this case report illustrates that even in patients with well-controlled HIV the diagnosis must be considered, and puts forward an unusual presentation in an otherwise asymptomatic patient. A 52 year old woman presented for a routine HIV follow-up appointment and was found to be experiencing weight loss. An abdominal examination revealed a right iliac fossa mass. Subsequent CT thorax, abdomen, pelvis imaging confirmed a large mass but did not allow determination of the primary source. Serological tumour marker investigations were unyielding. Trans-vaginal ultrasound guided biopsy of the mass demonstrated diffuse large B-cell lymphoma. This case report emphasises the importance of having a high index of suspicion for these cancers even in patients with low viral load who are on anti-retroviral treatment. It also demonstrates the importance of taking a multidisciplinary approach to diagnosis of the condition to enable prompt treatment and thus improve the outcome for the patient.