Background. Serology is the mainstay for syphilis diagnosis and treatment monitoring. We investigated serological response to treatment of syphilis according to disease stage and HIV status. Methods. A retrospective cohort study of 264 patients with syphilis was conducted, including 90 primary, 133 secondary, 33 latent, and 8 tertiary syphilis cases. Response to treatment as measured by the Venereal Disease Research Laboratory (VDRL) test and a specific IgM (immunoglobulin M) capture enzyme-linked immunosorbent assay (ELISA; Pathozyme-IgM) was assessed by Cox regression analysis. Results. Forty-two percent of primary syphilis patients had a negative VDRL test at their diagnosis. Three months after treatment, 85%-100% of primary syphilis patients had reached the VDRL endpoint, compared with 76%-89% of patients with secondary syphilis and 44%-79% with latent syphilis. In the overall multivariate Cox regression analysis, serological response to treatment was not influenced by human immunodeficiency virus (HIV) infection and reinfection. However, within primary syphilis, HIV patients with a CD4 count of <500 cells/μL had a slower treatment response (P = .012). Compared with primary syphilis, secondary and latent syphilis showed a slower serological response of VDRL (P = .092 and P < .001) and Pathozyme-IgM tests (P < .001 and P = .012). Conclusions. The VDRL should not be recommended as a screening test owing to lack of sensitivity. The syphilis disease stage significantly influences treatment response whereas HIV coinfection only within primary syphilis has an impact. VDRL test titers should decline at least 4-fold within 3-6 months after therapy for primary or secondary syphilis, and within 12-24 months for latent syphilis. IgM ELISA might be a supplement for diagnosis and treatment monitoring.