The incidence of serum antibodies to Chlamydia trachomatis and the possible association of a positive serology on semen parameters in 120 subfertile men were studied. The findings were compared with those of 120 fertile volunteers fathering two or more children. The two groups were divided into those without and those with a history of genito-urinary infection. There was no significant difference (p greater than 0.1) in the prevalence of antibody testing between the subgroups of fertile population. A statistically significant difference in the incidence of antibodies was found between the two subgroups of subfertile population. No significant difference was noted in the semen parameters between Chlamydia-positive and Chlamydia-negative fertile and subfertile patients.