Septic arthritis due to Mycobacterium kansasii is rare; only 40 cases have been published. A French national inquiry revealed the occurrence of 10 new cases between 1992 and 1997 (8 men and 2 women: mean age, 37 years; range, 25-54 years). Seven had an underlying condition: AIDS (n=4), chronic skin psoriasis and AIDS (n=2), or a renal transplant (n=1). Trauma to the joint, use of intra-articular corticosteroid(s) 1 month to 2 years after the event, and chronic skin psoriasis were risk factors. The mean interval between appearance of the first symptoms of arthritis and the diagnosis was 5 months. Monarthritis was localized to the knee (n=4), wrist (n=3), finger (n=1), elbow (n=1), or ankle (n=1). The main diagnostic procedure was culture of a synovial biopsy specimen. In all cases, debridement was associated with antimycobacterial treatment. Three patients died of AIDS during treatment, and another is still undergoing treatment; the other 6 patients were cured. M. kansasii infection should be considered in all cases of indolent arthritis with any of the following risk factors: local trauma, local or systemic corticosteroid therapy, chronic skin psoriasis, and immunodepression, especially that due to human immunodeficiency virus infection.