The prevalence of 23 HLA antigens were examined in 21 patients with a typical Reiter's disease, 21 patients with incomplete Reiter's syndrome as well as 16 patients with non-gonococcal urethritis. HLA-B 27 antigen was found in 85,7% of patients with typical Reiter's disease as compared to 7,4% of 413 healthy controls. HLA-B 27 was also detectable in 57,1% of patients with clinical suspicion of Reiter's disease. No significant difference from the control frequencies of HLA specifities was found in patients with non-gonococcal urethritis. The association between HLA-B 27 and Reiter's syndrome is so marked that tissue typing is a valuable adjunct in the diagnosis of Reiter's syndrome.