The case notes of 946 patients with primary and 854 with secondary syphilis who attended the department of genitourinary medicine at the Middlesex Hospital in 1965-84 were analysed retrospectively. Most patients were homosexual men, and the proportion of homosexuals showed a significant (p less than 0.0005) upward trend during the study period. There was also a significant upward trend in the percentage of British attenders (p less than 0.0001), as well as an increase in patients in social classes I, II, or III non-manual (p less than 0.0001). Homosexual men had a greater number of sexual partners than heterosexuals or women, and were also more likely to have had a sexually transmitted disease. The proportion of homosexuals with multiple partners showed a significant (p less than 0.0005) upward trend during the study period.