TT virus (TTV) is a novel DNA virus that has been identified in patients with post-transfusional hepatitis of unknown aetiology. However, its pathogenic role in liver injury remains unclear. To determine its frequency and clinical impact in cryptogenic liver diseases, we investigated the TTV prevalence in patients with liver enzyme elevations of unknown aetiology and in healthy subjects. Fifty-four patients (33 male, 21 female) who have been followed up for elevated ALT/AST levels of unknown aetiology and 118 healthy subjects (99 male and 19 female) were included in the study. TTV DNA was investigated by the polymerase chain reaction. Other possible causes of transaminase elevation were excluded in detailed biochemical and serological tests. A liver biopsy was performed in 45 patients. TTV DNA was detected in 46 patients with liver enzyme elevations (85.1%) and in 94 healthy subjects (79.6%). There was no statistical difference between the groups (p = 0.51). Histological examination of the liver revealed no specific change in TTV DNA positive patients that could be attributed to this virus infection. These results showed that TTV is a common virus in patients with liver enzyme elevation of unknown aetiology and even among healthy subjects in our geographical area. TTV infection is therefore widespread in the general population and does not seem to be associated with liver damage.