Twenty-two azoospermic males with no hormonal disturbances who revealed tubular spermatogenesis in testicular biopsy, were further surgically explored. Eight of these patients did not show any obstruction of the seminal ducts. This paradoxical finding is explained by the possibility of a low sperm production rate much below the absorption rate of sperm, mainly by the epididymis. A threshold theory is suggested in which sperm appear in the ejaculate only when their production rate is above a certain level. This threshold cannot yet be expressed quantitatively and there is no other way except through surgical exploration by which real obstruction can be differentiated from low sperm production rate in azoospermic males with tubular spermatogenesis.