We describe the first documented case of Mycoplasma felis infection in a woman who had common variable immunodeficiency and who presented with septic arthritis of the left hip and right knee. M. felis was isolated from both joints. She had been exposed to cats before the diagnosis of M. felis septic arthritis was made. Both of the patient's joints were surgically debrided, and she was treated with doxycycline for several months. In spite of initial improvement, destruction of her hip was noted. Subsequently, she underwent hip arthroplasty; histopathological examination of the bone at the time of surgery showed chronic osteomyelitis, and doxycycline therapy was continued.