PIP: 38 men who had contraceptive failure subsequent to vasectomy were examined to investigate causes of failure. In the semen specimen of 21 cases, sperm was observed in 20. 70 biopsies of the vas deference were taken from the 38 cases. Lumen clogging and scar tissue were observed under microscope in 9 biopsy sections. Thoroughly opening of lumen and masses, of sperm were observed in 17 sections. In 44 sections, numerous tiny channels formed inside the clogged lumen. Most of the channels had sperm in it. The formation of microchannels was believed to be associated with (1) loose tightening of vas at the vasectomy procedure. (2)Over-tightness that broke the vas, in which lumen opening was exposed. (3)Tying of the cut vas stopped local blood circulation which resulted in burst of tubes at nearby location. (4)Increased pressure inside lumen after vasectomy near the testicle end forced the tying thread to cut the tube open. The growth of membranal tissue gradually formed channels into lumen and leaked sperm from the tube opening. 2 procedures can be used to prevent the subsequent vas recanalization after vasectomy. First, for the reasons described previously, the possibility of tube bursting or opening always exists at the near end of testicle. Electric or chemical cauterization for about 1 millimeter on the opposite cut end would achieve satisfactory blocking. Second, burying one end of the cut vas deference under external membrane and properly sew it up is another reliable way to prevent potential recanalization.