About 2-2 1/2 million people do, or will repeatedly, participate in ano-rectal sexual intercourse in Britain alone. The anus and the rectum of these individuals are thus vulnerable to all the sexually transmitted diseases (except trichomoniasis). Male homosexuals appear to be more prone to these conditions than female heterosexuals, possibly because a large minority is indiscriminately promiscuous. Over the last 20 years homosexually acquired infections--particularly syphilis--have increased in Britain; these are now more readily admitted to because of changes in the law and in public opinion. The frequent changes of sexual partner among male homosexuals is the most notable epidemiological factor in the spread of venereal disease.