The methods of diagnosis (dark ground microscopy and serology), treatment, and follow up of 946 patients with primary and 854 with secondary syphilis who presented to a London STD clinic between 1965 and 1984 were reviewed retrospectively. On dark ground microscopy spirochaetes typical of Treponema pallidum were seen in 673 (78%) of 884 patients with primary syphilitic chancres. Of the patients with primary syphilis, 137 (14.5%) had negative serology results at presentation. Eight (0.9%) of the patients with secondary syphilis had negative results at presentation, but seven of these gave positive results one month later. Procaine penicillin was the treatment used most, and erythromycin the commonest alternative. The Jarisch-Herxheimer reaction occurred more often after treatment with penicillin than with erythromycin or tetracycline (p less than 0.005). In most patients the Venereal Diseases Research Laboratory (VDRL) test showed a consistent fall in titre after treatment; a small proportion, however, continued to give positive results (some at a high titre) with no other evidence of reinfection or treatment failure.