Altogether 52 children suffering from reactive arthritis were placed under observation. In 45 patients arthritis developed after nasopharyngeal infection and was marked by a favourable course with a rapid positive dynamics. 7 children with a history of intestinal infection manifested Reiter's syndrome; in these children arthritis was characterized by a grave and prolonged course and by a high laboratory activity. HLA A10 (RR = 2.7), B27 (RR = 2.5) and DR2 (RR = 2.19) antigen were detected significantly more frequently in ReA patients who suffered nasopharyngeal infection. In children with Reiter's syndrome, the genetic markers of predisposition to the disease were HLA A10 (RR = 5.93) and B27 (RR = 149.1) antigens.