AimThe appendix testis (AT) is a vestigial remnant of Müller's paramesonephric duct. Insulin‐like 3 hormone (INSL3) is produced in the Leydig cells of the testis. We investigated the possible correlation between AT length and plasma INSL3 concentrations in patients with congenital cryptorchidism (CCO) and patients with hydrocele, who served as controls.MethodsA total of 40 patients with CCO and 34 patients with hydrocele and orthotopic testes were investigated. Sixteen patients presented high cryptorchidism and 24 low cryptorchidism. During surgery, AT was identified in 34 patients with CCO (high cryptorchidism:15, low cryptorchidism:19) and 28 controls. Plasma INSL3 levels were measured with a spectrophotometry enzyme immunoassay Elisa sandwich technique.ResultsAT was present in 85.0% of the boys with CCO and 82.4% of the controls. A significant positive correlation was found between the AT length and INSL3 concentrations in CCO patients.ConclusionsA longer AT may reflect better testicular function in boys with CCO, since it is correlated with higher INSL3 concentrations.