After investigation of 150 patients with reactive arthritis and undifferentiated spondyloarthropathies, inflammation signs of urogenital tract (uretritis in men and endocervicitis in women) were found in about 2/3 of patients. In those patients with reactive arthritis and undifferentiated spondyloarthropathies Ureaplazma urealyticum was found in more than 1/4 of patients, Mycoplasma hominis - in 1/10 of patients, and both microbes appeared in 1/10 of patients. Micoplasmal infection occurred in equal frequency in patients with reactive arthritis and undifferentiated spondyloarthropathies with a tendency to appear more in women than in men.