During recent years several studies have suggested that a slight increase in the number of CAG repeat sequences in exon 1 of the androgen receptor gene causes idiopathic oligozoospermia. We tested whether CAG repeats are more numerous in men with idiopathic infertility compared to those with known causes of oligozoospermia. CAG repeats were analysed in a consecutive sample of 217 infertile men covering a wide range of diagnoses and sperm counts. Data were compared with those of a control group of 131 normozoospermic men including 62 fathers. CAG repeats (x +/- SD) did not differ between idiopathically (21.4 +/- 2.9) and non-idiopathically infertile men (21.6 +/- 2.8) or normozoospermic men of unproven fertility (20.6 +/- 3.0). Only fathers had significantly fewer repeats (19.4 +/- 3.1; p < 0.001). Different from controls, no correlation between CAG repeats and any semen parameter existed in patients. Comparison of our and published studies showed that odds ratios for infertility in men with CAG repeat length in the upper quartile of the normal range increased when the controls were selected by proven fertility. We conclude that more numerous CAG repeats do not directly cause oligozoospermia and propose that men with longer CAG repeats might be more prone to develop infertility in response to any pathogen/epigenetic factors.