A 78-year-old man with metastasised prostate carcinoma presented with a painless paraparesis. His cerebrospinal fluid showed elevated protein and a mononuclear pleiocytosis, but cytology investigations of 5 separate samples revealed no malignant cells in the cerebrospinal fluid. Extensive viral and bacterial tests (including ELISA for Borrelia burgdorferi) of serum and cerebrospinal fluid were negative. On the day radiation therapy for presumed leptomeningeal metastases was due to start the IgG and IgM Western blot for Borrelia were found to be positive, indicating neuroborreliosis. Soon after the start of antibiotic therapy the paraparesis began to improve and after four weeks the patient had made a complete recovery. In patients with a progressive paraparesis, neuroborreliosis should be considered even in the absence of pain.