Hepatitis B virus (HBV) and hepatitis C virus (HCV) coinfection is often associated with severe forms of liver disease. However, comprehensive studies are lacking, and scant information is available regarding the virological behavior over time in coinfected patients. This study enrolled 133 untreated HBV/ HCV-positive patients (male/female = 102/31; median age 51 years [range: 22-83 years]) who were longitudinally followed up for 1 year with bimonthly evaluation of HBV/HCV viremia levels and liver biochemistry. Thirty of these patients had triple infection with hepatitis Delta virus (HDV), while 103 patients were HDV-negative. In the HDV-negative group, active infection with both HBV and HCV was revealed in 24 cases, inactive infection by both viruses was seen in 15 cases, active HBV/ inactive HCV was seen in 15 cases, and inactive HBV/active HCV was seen in 49 cases. However, 32 subjects (31%) presented dynamic virological profiles characterized by fluctuation of HBV and/or HCV viremia levels that at different time points were over or under the cutoff limits. Consequently, a correct diagnosis could be performed in these subjects only by serially repeating the virological tests 1 year apart. Similarly, 15 of the 30 HDV-positive subjects showed active HBV and/or HCV infection, with fluctuating virological patterns in 8 cases. In conclusion, this study showed that the virological patterns in HBV/HCV coinfection are widely divergent and have dynamic profiles. A careful longitudinal evaluation of the viremia levels of both viruses is essential for making a correct diagnosis and tailoring the appropriate therapeutic schedule in coinfected patients. However, comprehensive studies are lacking, and scant information is available regarding the virological behavior over time in coinfected patients. This study enrolled 133 untreated HBV / HCV-positive patients (male / female = 102/31; median age 51 years [range: 22-83 years]) who were longitudinally followed up for 1 year with bimonthly evaluation of HBV / HCV viremia levels and liver biochemistry. Thirty of these patients had triple infection with hepatitis Delta virus (HDV), while 103 patients were HDV-negative. In the HDV-negative group, active infection with both HBV and HCV was revealed in 24 cases, inactive infection by both viruses was seen However, 32 subjects (31%) presented dynamic virological profiles characterized by fluctuation of HBV and / or HCV viremi a, that levels at at different time points were over or under the cutoff limits. Consequently, a correct diagnosis could be performed in these subjects only by serially repeating the virological tests 1 year apart. Similarly, 15 of the 30 HDV- and / or HCV infection, with fluctuating virological patterns in 8 cases. In conclusion, this study showed that the virological patterns in HBV / HCV coinfection are widely divergent and have dynamic profiles. A careful longitudinal evaluation of the viremia levels of both viruses is essential for making a correct diagnosis and tailoring the appropriate therapeutic schedule in coinfected patients.