Reiter's syndrome (RS) is uncommon in children, and the classic triad manifestations of RS usually do not occur simultaneously in children. It is often clinically confused with other childhood illnesses. We report a case of RS in a 7-year-old boy with a family history of ankylosing spondylitis. He had developed intermittent arthralgia of the right knee for about 6 months and occasional bilateral eye pain for several months prior to admission. In the 5 days before admission, he developed multiple oral ulcers, weight loss from 25 to 22 kg and fever. Physical examination showed injected bilateral conjunctivae and the right knee joint with swelling, local warmth, and tenderness over the patellar ligament. Laboratory results revealed positive histocompatibility antigen-B27 (HLA-B27), negative rheumatoid factor (RF) and antinuclear antibody (ANA) and normal urinalysis. RS was diagnosed based on the findings of both arthritis and conjunctivitis. The arthritis was treated with acetaminophen and naproxen. In conclusion, juvenile RS should be considered in children with arthritis and conjunctivitis, positive HLA-B27, negative RF and ANA and a family history of related diseases.