The clinical story of a young woman with chronic erythema migrans followed by polyradiculoneuritis and recurrent oligoarthritis is reported. The story corresponds to the disease described by Steere et al. [7, 8, 9, 10] in 1976 and known in the U.S.A. as "Lyme's disease". The condition is epidemic and occurs during the summer. It begins with skin lesions characteristic of chronic erythema migrans, which are consecutive to tick bite. This is followed, a few days or weeks later, by neurological disorders (aseptic meningitis, encephalitis, cranial nerve paralysis and/or polyradiculoneuritis), transient and recurrent attacks or arthritis mostly in the larger joints and, occasionally, conduction disorders in the heart. The course of the disease is that of an inflammatory condition. The presence of immune complexes in the serum and synovial fluid is suggestive of local and systemic immune reaction to a hypothetically viral agent introduced by the tick bite. The fact that the incidence of DR W2 antigen is greater in patients with severe lesions suggests individual predisposition.