['To determine the frequency of C677T methylenetetrahydrofolate reductase (MTHFR) polymorphism in fertile and subfertile males, and the MTHFR-dependent response of sperm concentration after folic acid and/or zinc sulfate intervention.', 'Double-blind, placebo-controlled intervention study. Two outpatient fertility clinics and nine midwifery practices in The Netherlands.', 'One hundred thirteen fertile and 77 subfertile males. Daily capsules of folic acid (5 mg) and/or zinc sulfate (66 mg), or placebo for 26 weeks.', 'Prevalence of C677T MTHFR polymorphism and the response of sperm concentration related to MTHFR carriership after intervention treatment.', 'The C677T methylenetetrahydrofolate reductase genotypes were comparable in fertile and subfertile males. Independent of fertility state, sperm concentration significantly increased in wild-types after folic acid and zinc sulfate treatment only. Heterozygotes and homozygotes did not significantly benefit from either treatment.'] Does c677T methylenetetrahydrofolate reductase polymorphism interfere with the effects of folic acid and zinc sulfate on sperm concentration?