Two cases with HLA-B 27 negative, Chlamydia-induced reactive arthritis (ReA) were described. Case 1: A 30 y.o. male developed balanitis, urethritis, arthritis of both knees, elbows, shoulders and hip joints on May in 1997. Laboratory findings revealed CRP 2.7 mg/dl (normal range < 0.3), ESR 33 mm/h and negative rheumatoid factor (RF) test. Anti-Chlamydia trachomatis antibodies, IgG 2.22, IgA 3.33 were positive. HLA-typing revealed A 2, A 24 (9), B 39 (16), B 52 (5). He was diagnosed as ReA and arthritis subsided with treatment of minocycline and nonsteroidal antiinflammatory drugs (NSAIDs). Case 2: A 40 y.o. Iranian American male developed balanitis, urethritis, lumbago, arthritis of both elbows, knees and foot joints, iridocyclitis on August in 1995. Chlamydia trachomatis was detected in the urethral swab culture. He was diagnosed as ReA and treated with minocycline and NSAIDs. He was referred to our hospital on June in 1996. Arthritis at both knees and feet was detected. Laboratory findings revealed CRP 0.8 mg/dl, negative RF test was revealed. Antibodies to Chlamydia were positive (IgG 1.49, IgA 1.53) positive. HLA typing revealed A 1, A 2, B 37, B 55 (22). He was again treated with minocycline and NSAIDs and ReA ameliorated. Since HLA-B 22, B 37 and B 39 have been reported to cross-react or to have homology with B 27, B 22, B 37 and B 39 are likely to related to inducing ReA.