A total of 659 consecutive patients underwent venography for the evaluation of idiopathic left varicocele before sclerotherapy. In 484 cases no valves of the left spermatic vein could be demonstrated, while 172 patients demonstrated competent valves or absent insertions of the left spermatic vein at the typical point on the left renal vein plus retrograde flow over persistent intercardinal anastomoses. Stenosis of the renal vein occurred in 103 patients (15.7%). In only 3 patients was stenosis observed in the course of the left iliac vein, suggestive of the so-called distal nutcracker phenomenon. We propose that during embryogenesis disturbances in the development of the secondary venous system may result in the idiopathic left varicocele.