The use of the condom by male patients attending a clinic specialising in sexually transmitted diseases has been assessed over a period of six months. Condom users were divided into those who used them properly and invariably, and those who did not. In the group studied, correct use of the condom was associated with a significantly lower probability of acquiring gonorrhoea (P less than 0-001) and a significantly higher chance of there being no sexually transmitted disease diagnosed (P less than 0-0005). The diagnosis-rate of non-specific urethritis, however, did not differ among the groups.