Intravascular large B-cell lymphoma or intravascular lymphomatosis (IVL) is an extremely rare form of non-Hodgkin's lymphoma. The most common clinical sign is fever of unknown origin (FUO). Histologically, there is proliferation of malignant lymphoid cells within vascular lumina. Cytologically, the cells have features similar to those found in classical large cell lymphoma. Examination of pulmonary artery blood showed the presence of this abnormal population in our patient; to the best of our knowledge there are only four other reports of detection of circulating tumor cells in IVL. The outcome is very poor. The diagnosis is most frequently made after biopsy of skin or brain but is often established post mortem. We present what is – to our knowledge – the first reported case of IVL diagnosed after biopsy of a testicle. In the event of FUO and suspicion of a malignancy, IVL – although very rare – should be one of the differential diagnoses.