Sixty-four patients (43 men and 21 women) with acute viral hepatitis were subjected to clinical, clinical chemical and electroencephalographic examinations. A pathological EEG was found in 19 patients. The average age was the same in the entire group, in the group with normal EEGs and in the group with pathological EEGs (35.4, 35.8 and 35.2 years respectively). The proportion of HBsAg-positive to HBsAg-negative hepatitis in the groups with pathological and normal EEGs did not show any appreciable difference. Of the patients with pathological EEGs, 36.8% were drug addicts, and only 8.9% of these these with normal EEGs. A dependence of the EEG on the laboratory values SGOT, SGPT, prothrombin time or bilirubin was not found. In 14 of the 19 patients, there was a slight general alteration with irregular alpha rhythm and markedly increased 5--7 c/sec theta activity. In 5 patients, there was a moderately severe general alteration with predominant theta rhythm and interspersed delta waves. In 5 patients, the EEG normalized; it remained pathological in 14 patients. Even after exclusion of all other factors, such as drugs or hypertension, which might have led to a pathological EEG, we found pathological EEG findings in 15.6% of the 64 patients with acute viral hepatitis and no clinical encephalopathjy.