In this article, the authors described the role of mycoplasmas or ureaplasmas on acute epididymitis. Pathogenicities of mycoplasmas or ureaplasmas have not been confirmed for epididymitis. If we confirm the pathogenicity of some microorganisms for acute epididymitis, we have to take the specimens of epididymal fluid or tissue by aspiration or other methods, and have to detect these pathogens by nucleic acid amplification tests (NAAT) or culturing in these specimens. However, it is difficult for clinicians to take specimens from the epididymis of patients with high fever or scrotal swelling. When we diagnose chlamydial epididymitis, the detection of Chlamydia trachomatis from the urine specimens from patients with acute epididymitis is important. I have previously reported a case of a patient with acute epididymitis, which was thought to be caused by Mycoplasma genitalium (reference number 16). The patient was 30-year-old man who attended a clinic with high fever and scrotal swelling. He had sexual relations with a female commercial sex worker without a condom 20 days before the first visit. The right scrotum had painful swelling and the scrotum contents were palpable as a lump. The prostate was also swollen and tender. An increased number of white blood cells was shown in the urinary sediment of the first voided urine, but he had no symptoms of urethritis. A culture of urine was negative. From the urine specimen, Mycoplasma genitalium was detected by real-time polymerase chain reaction (PCR) for adhesion and PCR for 16S rRNA. Other pathogens including Neisseria gonorrhoeae, C. trachomatis, Ureaplasma urealyticum, Ureaplasma parvum or Mycoplasma hominis were not detected by NAAT. The patient was treated by minocycline orally for 6 days and with i.v. injection of cepharosporins for 3 days, but the scrotal swelling did not improve. Levofloxacin was added for 7 days and symptoms improved. M. genitalium was not detected from a urine specimen on the 14th day after the first visit. In our case, M. genitalium was the only microorganism detected from the urine specimen from the patients with symptoms and clinical courses of acute epididymitis. Of course, there was no clear evidence as describe above, but it was one of data which M. genitalium can be a pathogen for epididymitis. In this article, M. genitalium was detected from 5 patients (8.9%) with epididymitis and its prevalence was not so high. However, M. genitalium can rarely be detected from general population of men. To confirm pathogenicity of mycoplasmas or ureaplasmas for epididymitis, it is important to accumulate the prevalence data of these pathogens. For this reason, the data of this article are available.