Outbreaks of acute hepatitis C in HIV-positive men who have sex with men (MSM) are being reported in large cities in western countries along with increasing rates of sexually transmitted diseases. All HIV individuals attended at a large outclinic in Madrid within the last 5years were examined. Incident syphilis was diagnosed based on rapid plasma reagin (RPR) reactivity, being negative previously or showing >4-fold increase. Acute hepatitis C was diagnosed based on HCV antibody seroconversion and/or positive serum HCV-RNA after being negative within the last year. A total of 859 episodes of syphilis and 19 of acute hepatitis C were diagnosed during the study period. Syphilis was recognized in 65/2,094 (3.1%) individuals attended in 2008 and rose up to 261/2,512 (10.4%) in 2012 (P<0.001). Acute hepatitis C was diagnosed in only one subject in 2008 but rose up to 7 in 2012 (P=0.12). All 19 HIV patients with acute hepatitis C were MSM. Syphilis was diagnosed concomitantly in seven. All eight individuals who were treated with peginterferon/ribavirin were cured, whereas only one untreated experienced spontaneous clearance (P=0.004). Two clusters of infections by HCV genotypes 4 and 1a were identified by phylogenetic analyses. The incidence of acute hepatitis C is low but steadily increasing in HIV-positive MSM in Madrid (<1% yearly), despite the very high rates of syphilis (currently 20% yearly in HIV-positive MSM). Preventive measures for sexually transmitted infections and periodic HCV screening are warranted in this population as treatment of acute hepatitis C is very effective.