Infertility in subjects affected by undesceded testis occurs in from 25 to 100% of the cases according to the various Authors. This depends on whether one or both gonads are concerned and on the age when medical and/or surgical treatment of the condition was begun. Our study was made in two successive periods: first we studied the secretory causes of infertility by examining some histological parameters, (MTD, IFT), and endocrinological ones, (basal testosterone and then after HCG, basal gonadotropin and then after GnRH) in 43 subjects of ages ranging between 2 and 13 years who are affected by uni- or bilateral maldescension. We then analysed the excretory causes of infertility and classified and interpreted 108 epididymo-testicular malformations out of a total of 144 undescended testis observed during our last year of work. We conclude that from a histological point of view, from the third year after birth there is a steady progressive reduction in the tubular fertility index in undescended testis compared to scrotal testis. There is no significant reduction, however, in the mean tubular diameter up to the prepubertal phase. From the endocrinological point of view, we found a normal LH secretion and hypertonia in the pituitary secretion of FSH in bilateral maldescension, not found, however, in unilateral maldescension. Finally, anomalies concerning the epididymo-testicular relationship are found in 75% of undescended testis and of these 36% have a definite effect on infertility of the excretory type.