Spondyloarthritis (SpA) includes reactive arthritis (ReA) and enteropathic arthritis (EA), which are clinically important but often misdiagnosed. ReA, sterile inflammatory arthritis, arises after certain genitourinary or gastrointestinal infections. Chlamydia are the most common pathogens causing ReA; ReA due to Chlamydia infection is called Chlamydia-associated arthritis (Chl-AA). Recently, Chlamydia trachomatis was detected in the synovial tissue from patients with ReA by electronmicroscopy. In addition, mRNA as well as DNA has been detected in the synovial tissue, suggesting that Chlamydia are viable in inflamed joints. Thus, the notion that ReA is a sterile inflammation should be reconsidered. Chl-AA patients, especially women, often show no symptoms and signs of genitourinary infection. Thus, Chl-AA should be suspected in patients with inflammatory arthritides that is difficult to diagnose. EA is accompanied by inflammatory bowel diseases (IBD). In Japan, over 130,000 individuals have IBD; IBD is diagnosed in 6,500 individuals every year. Around 10% IBD patients develop arthritis, suggesting that 13,000 patients develop arthritis every year. SpA includes peripheral and axial arthritis; axial arthritis includes spondylitis and sacroiliac arthritis. Sacroiliac joint tests need to be performed to diagnose sacroiliac arthritis. Rheumatologists should be aware of the pathogenesis of Chl-AA and EA and diagnose and treat these diseases appropriately.