In the late twentieth century, there was significant increase in the incidence of syphilis in different countries of Europe, USA, and Australia. In Russia, more than 2.5 million people suffered from syphilis during an epidemic in 1990—2000. Currently, incidence pattern of syphilis is still characterized by increased number of latent and late forms along with decrease in the overall incidence of the disease. Errors in the diagnosis of syphilis infection are of particular importance. The paper presents two clinical cases of unspecified (either early or late) latent syphilis, in the females who sought medical advice of obstetrician-gynecologist in connection with infertility. In these cases, doctors relied on the negative results of non-treponemal tests (e.g., microprecipitation reaction), ignoring the positive results of treponemal tests (passive hemagglutination test), which led to delayed diagnosis of syphilis.