Background. A decline of hepatitis C virus (HCV) antibody titers (anti-HCV), ultimately resulting in seroreversion, has been reported following clearance of viremia in both acute and chronic HCV infection. However, frequency of seroreversion remains unknown in human immunodeficiency virus (HIV)/HCV-coinfected patients. We describe anti-HCV dynamics among HIV-infected men who have sex with men (MSM) following acute HCV infection and reinfection. Methods. Primary acute HCV infection was assumed when a subject was anti-HCV negative prior to the first positive HCV RNA test. Anti-HCV was measured at least annually in 63 HIV-infected MSM, with a median follow-up of 4.0 years (interquartile range IQR, 2.5–5.7 years). Time from HCV infection to seroconversion, and from seroconversion to seroreversion, was estimated using the Kaplan-Meier method. Longitudinal anti-HCV patterns were studied using a random-effects model to adjust for repeated measures. Results. Median time from HCV infection to seroconversion was 74 days (IQR, 47–125 days). Subjects who cleared HCV RNA (n = 36) showed a significant decrease in anti-HCV levels (P<.001). Among 31 subjects with sustained virologic response (SVR), anti-HCV became undetectable during follow-up in 8; cumulative incidence of seroreversion within 3 years after seroconversion was 37% (95% confidence interval, 18%–66%). Eighteen subjects became reinfected during follow-up; this coincided with a subsequent increase in anti-HCV reactivity. Conclusions. A decline of anti-HCV reactivity was associated with HCV RNA clearance. Seroreversion was very common following SVR. Upon reinfection, anti-HCV levels increased again. Monitoring anti-HCV levels might therefore be an effective alternative for diagnosis of HCV reinfection.