A 66-year-old woman presented with severe shooting pains throughout her back and legs, followed by progressive deafness, weight loss and headache. She had a history of marginal zone B-cell lymphoma stage IV-B, for which she was successfully treated with immunochemotherapy and rituximab maintenance therapy. A relapse was suspected, but chemotherapy was not administered, since, despite elaborate investigations, malignancy could not be proven. Because of a history of tick bites she was tested for antibodies against Borrelia burgdorferi in serum and cerebrospinal fluid (CSF), which were negative. However, a B burgdorferi PCR on CSF came back positive. The patient was treated for seronegative Lyme neuroborreliosis with ceftriaxone intravenously and dramatically improved. This case presentation demonstrates that, in immunocompromised patients, it is important not to solely rely on antibody testing and to use additional diagnostic tests to avoid missing or delaying the diagnosis.