The animal epididymis was known to play an essential role in the acquisition of motility and fertilising capacity of testicular spermatozoa. Little was known about the function of the human epididymis and this paper describes studies arising from tissues obtained at the time of surgical procedures on the epididymis and vas deferens. Even though the human epididymis differs from that of other animals in fine structure and luminal contents, its function is similar in that spermatozoa gain motility and fertilising capacity during their passage through it. The poor fertilising capacity (24% of 76 cycles) and 2 (2.6%) conceptions with spermatozoa from an obstructed epididymis or vas deferens compared with a fertilisation rate of 88% (pregnancy in 37.5%) in men without obstruction is evidence of this. The defect in fertilising capacity may be overcome by using the technique of intracytoplasmic sperm injection, where fertilisation and embryo transfer occurred in 95% of the 38 couples with obstruction and 34% of the wives conceived.