To investigate the role of the cation channel of sperm 1 (CatSper1) protein in the pathogenesis of idiopathic asthenozoospermia.Sperm samples from patients with idiopathic asthenozoospermia were separated by Percoll discontinuous density gradients, and the distribution and expression of the CatSper1 protein were determined by immunocytochemistry. Western blotting was used to detect the different expressions of CatSper1 in the ejaculated sperm from the normal control, mild asthenozoospermia, moderate asthenozoospermia and severe asthenozoospermia groups, followed by statistical analyses.The expression of CatSper1, located in the principle piece of the sperm tail, was reduced significantly in the samples from the idiopathic asthenozoospermia patients as compared with the normal controls (t = 2.188, P = 0.042). The relative contents of the CatSper1 protein in the sperm of the control, mild asthenozoospermia, moderate asthenozoospermia and severe asthenozoospermia groups were 0.806 +/- 0.266, 0.669 +/- 0.207, 0.505 +/- 0.214 and 0.295 +/- 0.162, respectively, significantly decreased in the asthenozoospermia patients in comparison with the normal controls (P <0.05). There was a positive correlation between the percentage of progressively motile sperm and the relative content of the CatSper1 protein (r = 0.633, P = 0.000).The decreased or abnormal expression of the CatSper1 protein may be a factor involved in the pathogenesis of idiopathic asthenozoospermia.