We report a case of Lyme arthritis in a 18-year-old woman, who presented to the Unit of Infectious Diseases of our hospital, complaining of joint pain for seven months. During the hospitalization, she presented an increase of erythrocyte sedimentation rate, a right maxillary sinusitis and inflammation of hands, wrists and right ankle, with joint effusion shown by MRI; IgM for Borrelia burgdoferi were positive by both ELISA test and Western blot. The patient was treated with a first cycle of Amoxicillin for seven days, followed by Penicillin for 21 days. During the next months, she had a relapse of the original symptoms and required a second cycle of antibiotic therapy with penicillin and steroids for 21 days.