Bilateral vasectomy was performed in 30 men. The vasa were tied with radio-opaque ligatures and X-rays taken to assess the separation of the ends. There was good correlation between the length of vas excised and the resulting separation of the markers. In order to achieve a separation greater than that of sperm granulomas associated with vasectomy failure, at least 7 cm of vas would have to be removed. However, excision of this length cannot be recommended because of subsequent difficulties with vasovasostomy. It is suggested that as well as excising a length of vas during vasectomy, a second method is also used to prevent spontaneous recanalisation.