Objective To analyze the changes in peripheral blood monocyte subpopulations in patients with primary, secondary and latent syphilis. Methods Flow cytometry was used to detect CD14highCD16- and CD14+CD16+ monocyte subpopulations in peripheral blood from 58 patients with untreated syphilis, including 36 cases of latent syphilis,8 cases of primary syphilis and 14 cases of secondary syphilis, as well as from 65 normal human controls. Restflts Compared with the normal controls, the proportion of CD14+CD16+ monocytes among total monocytes was significantly elevated (12.0% ± 5.0% vs 6.0% ± 3.3%, t = 7.25, P < 0.01), while that of CD14highCD16- monocytes was down-regulated (88.0% ± 5.1% vs 94.0% ± 3.5%, t = -7.20, P < 0.01). No statistical difference was observed in the proportion of CD14+CD16+ or CD14hhighCD16- monocytes among the patients with primary syphilis, secondary syphilis and those with latent syphilis (all P > 0.05). Conclusions The changes in peripheral blood monocyte subpopulation in patients with untreated syphilis may be associated with the permanent infection of Treponema pallidum, but have no obvious correlation with clinical stage of syphilis.