Summary: A case of Reiter's syndrome occurring in an 11-year-old, pre-pubertal boy is described. The boy was a heterozygote for the histocompatibility antigen B27 and other arthritic members of his family included his mother with colitic arthritis and an aunt with ankylosing spondylitis. His HLA-B27 negative sibs have remained well. Shigella, Salmonella and Yersinia organisms have been previously incriminated as precipitating factors in some patients with Reiter's syndrome but no evidence of recent infection with any of these agents was found in this patient. The case is reported because of the rarity of the condition at this age.