Background: Infection with Borrelia burgdorferi sensu lato complex (B. b. sl) spirochetes can cause Lyme borreliosis, manifesting as localized infection (e.g. erythema migrans) or disseminated disease (e.g. Lyme neuroborreliosis). Generally, patients with disseminated Lyme borreliosis will produce an antibody response several weeks post-infection. So far, no case of neuroborreliosis has been described with persistently negative serology one month after infection. Case presentation: We present a patient with a history of Mantle cell lymphoma and treatment with R-CHOP (rituximab, doxorubicine, vincristine, cyclofosfamide, prednisone), with a meningo-encephalitis, who was treated for a suspected lymphoma relapse. However, no malignant cells or other signs of malignancy were found, and microbial tests did not reveal any clues, including Borrelia serology. He did not recall being bitten by ticks, and a Borrelia PCR on CSF was negative. After spontaneous improvement of symptoms, he was discharged without definite diagnosis. Several weeks later, he was readmitted with a relapse of symptoms of meningo-encephalitis. This time however, a Borrelia PCR on CSF was positive, confirmed by two independent laboratories, and the patient received ceftriaxone upon which he partially recovered. Interestingly, during the diagnostic process of this exceptionally difficult case, a variety of different serological assays for Borrelia antibodies remained negative. Only P41 (flagellin) IgG was detected by blot and the Liaison IgG became equivocal 2months after initial testing. Conclusions: To the best of our knowledge this is the first case of neuroborreliosis that is seronegative on repeated sera and multiple test modalities. This unique case demonstrates the difficulty to diagnose neuroborreliosis in severely immunocompromised patients. In this case, a delay in diagnosis was caused by broad differential diagnosis, an absent known history of tick bites, negative serology and the low sensitivity of PCR on CSF. Therefore, awareness of the diagnostic limitations to detect Borrelia infection in this specific patient category is warranted.