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Objective: To investigate the frequency of mutations of the cystic fibrosis transmembrane regulator (CFTR) gene in males with reduced sperm quality before intracytoplasmic sperm injection (ICSI)., Design: The nine most frequent cystic-fibrosis-causing mutations in the German population and IVS8T alleles were analyzed., Setting: University-based centers for reproductive medicine and clinical genetics., Patient(s): An unselected group of 597 males with oligo-, astheno-, terato-, crypto-, oligoasthenoteratozoospermia, or azoospermia, which underwent pre-ICSI genetic counseling over a 5-year period., Intervention(s): Blood samples were collected from the patients during genetic counseling., Main Outcome Measure(s): Frequency of mutations of CFTR gene in infertile males., Result(s): A heterozygous CFTR mutation was observed in 34 of 597 patients (5.70%). None of the patients had two CFTR mutations. Given that our mutation panel recognizes about 82% of heterozygotes, it can be assumed that the frequency of CFTR heterozygotes in our cohort is about 6.94%. The frequency of CFTR mutations in our cohort did not correlate with a reduced sperm count., Conclusion(s): The frequency of cystic fibrosis in the German population is 1:3300. Thus, a CFTR heterozygosity of 3.42% can be estimated. This indicates that in our cohort of infertile males, the frequency of CFTR heterozygosity is twofold higher than in the general population (P<.0001).