A 54-year-old housewife became ill with transient diplopia followed by facial paralysis, tremor of the right hand and a progressively worsening disturbance in gait. Four years after the first appearance of the disease bilateral hypacusia, paralysis of the right hypoglossal nerve, dysarthria, severe spastic tetraparesis with ataxia, urinary retention and an organically induced character change were present. Lumbar puncture fluid contained 51/mm3 lymphocytes and 1164 mg/dl protein. Immunoglobulin A, G and M, synthesised locally, could be detected in cisternal fluid. The serum had a high antibody titre against Ixodes ricinus-Borrelia antigen and the same antibodies, formed locally, were present in the lumbar fluid. The neurological deficiencies improved markedly under parenteral penicillin treatment and the antibody titre decreased. The detection of Borrelian antibodies, especially those synthesised locally in the CSF, enabled an aetiological diagnosis to be made in seven other patients aged 18-47 years in whom the disease had been present for 1 to 8 years. These patients likewise presented symptoms characteristic of a disseminated disease of the CNS, with spastic pareses and evidence in the spinal fluid of chronic inflammation and disruption of CSF barriers in particular. This progressive encephalomyelitis differs from the common and spontaneously healing meningo-polyneuritis (Garin-Bujadoux-Bannworth), the usual manifestation of erythema chronicum migrans of the nervous system, in its progressive nature, its invasion of the CNS and the possible long lasting severe damage when not specifically treated.