The etiological role of Chlamydia trachomatis in chronic nonbacterial prostatitis is controversial. To assess whether C. trachomatis relates to chronic nonbacterial prostatitis, we investigated the serum levels of immunoglobulin G (IgG) and IgA antibodies specific for C. trachomatis in 41 patients with chronic nonbacterial prostatitis and 47 patients with prostatodynia. The positive rates of the IgG antibody were 29% and 28% in patients with chronic nonbacterial prostatitis and prostatodynia, respectively. There was little difference in the positive rates of the IgG and IgA antibodies for C. trachomatis, between patients with chronic nonbacterial prostatitis and prostatodynia. Also, there was no significant difference in the positive rate of the IgG antibody between 51 patients complaining of urinary symptoms and 25 complaining of referred pain. However, a high positive rate of the IgG antibody (80%) was observed in patients with hematospermia. Our findings suggest that C. trachomatis could not be etiologically responsible for chronic nonbacterial prostatitis, but that it may be related to hematospermia.