Sterilisation (vasectomy in the case of the male and tubal ligation or laporoscopy or other variants in the case of the female) is a common method of contraception. It is perceived as having none of the practical and aesthetic disadvantages of ‘mechanical’ methods and none of the health risks associated with oral contraception. It seems to be generally assumed that these procedures are one hundred per cent effective. Indeed, they probably are the most effective methods available, but there are at least two possible sources of failure. First, the procedure may not be performed correctly, though in the case of vasectomy this does not present a very serious problem if, as is normal vasectomy practice in this country, fertility tests are carried out over a fairly extended period before the patient is advised that he may rely upon the operation.