To study the association between plasma antibodies to Chlamydia trachomatis and male infertility, 90 men from infertile couples attending a University Hospital IVF clinic for IVF/intracytoplasmic sperm injection, and 190 healthy blood donors as control subjects were studied for IgG and IgA antibodies to C. trachomatis, and for the men from infertile couples seminal fluid analysis was performed according to the World Health Organization criteria. The prevalence of plasma IgG antibodies to C. trachomatis was higher among men from infertile couples than control men, and men with chlamydial antibodies had lower sperm counts than those without.