Our findings suggest the involvement of the overexpression of the AQP1 protein in individual capillary endothelial cells in the tunica vaginalis in the development of hydrocele testis, because the level of expression of the AQP1 protein in the tunica vaginalis is higher in patients with primary hydrocele than in controls, while the densities of AQP1-expressing capillaries are similar between them. Here, AQP1 is linked to Hydrocele testis.