The hepatitis C infection is highly prevalent among HIV-infected patients. As a direct consequence of the increased survival of these patients in the HAART era, liver disease and its long-term complications have became a genuine health problem in these patients. The treatment of chronic HCV hepatitis is associated with several secondary effects, hiperlactacidemiae/lactic acidosis is one of the most dangerous. It appears to be related with the association of ribavirin and ddI, d4T or AZT. These are three cases of hiperlactacidemiae/lactic acidosis collected during the first twelve months of treatment with pegylated interferon and ribavirin in University Hospital of Guadalajara.