The number of (trinucleotide) CAG repeats within the androgen receptor (AR) gene is inversely correlated with transcriptional activity of testosterone-target genes. Although abnormally long CAG repeats are strongly associated with male infertility, it is unclear whether CAG repeat length polymorphism can affect androgen receptor activity and sperm parameters. To explore the previously suggested association between CAG repeats and male fertility, we conducted this prospective cohort study.We enrolled 172 men attending the IVF unit in Shaare-Zedek Medical Center. Sperm concentration, motility and morphology and the number of CAG repeats in the AR gene were measured.Mean CAG repeat length was greater in teratozoospermia (<14% normal forms, strict criteria) than in the normal morphology group [mean +/- 95% confidence interval (CI) 22.19 +/- 0.38 versus 21.25 +/- 0.28, P = 0.02]. Logistic regression models showed that longer CAG repeats were associated with abnormal sperm morphology [odds ratio for percentage of normal forms per unit increase in CAG repeat length 1.14 (95% CI 1.01-1.28), P = 0.04]. No association was found between CAG repeat length and sperm concentration or motility.We found a positive correlation between CAG repeat length and teratozoospermia. This finding validates the concept that AR function is inversely regulated by length of its CAG repeat tract.