The diagnosis of azoospermia or severe oligospermia that is made using conventional techniques (testicular biopsy, epididymal exploration, and vasovesiculography) may in some cases remain a dilemma. In such circumstances, post‐testicular causes of obstruction must be evaluated. Following the clinical experience acquired in a selected population of 150 severely infertile subjects, the total sperm count in the fluid obtained from the bladder after a seminal tract washout during vasovesiculography has proved to be a valid tool to diagnose the location of the (sub‐)obstruction in the seminal tract in complicated cases.Some representative cases are presented. In particular, seminal tract washout (STW) helps to identify functional distal seminal tract emptying disturbances and epididymal incomplete obstruction.