Chronic asthenia syndrome has been associated with Epstein-Barr virus (EBV). To investigate the possible role in this syndrome of human herpesvirus type 6 (HHV-6), a new herpesvirus discovered in 1986, we performed a serological study of 44 patients diagnosed of chronic asthenia syndrome and 50 controls. Fifty healthy controls (34%) and 44 patients (54.5%) had positive serological tests for HHV-6. This difference was significant (p less than 0.05). The rate of positive serological studies for CMV and EBV was higher in healthy controls (n = 50) than in patients (n = 37) (96% vs 67.7% for CMV [p less than 0.001] and 92% vs 78.4% for EBV [non significant difference]). We conclude that HHV-6 may play a role in the pathogenesis of chronic asthenia syndrome. In our group of patients there were no significant differences in the presence of anti EBV capsid antibodies.