The aetiologic types of sporadic acute viral hepatitis in 169 pregnant women were compared with those of 70 non-pregnant women and 287 adult men. The majority of pregnant women (87.6%) came with acute hepatitis in the last trimester of pregnancy. Non-A, non-B (NANB) hepatitis accounted for 81.6% of hepatitis during pregnancy in comparison with 48.6% in non-pregnant women and 57.1% in adult men. Hepatitis A was extremely uncommon during pregnancy. Hepatitis B infection accounted for 17% of all cases in pregnant women compared with 45% in controls. Acute viral hepatitis in pregnancy had a poor outcome as assessed by maternal and/or fetal mortality (28.5%). The outcome was equally bad in hepatitis NANB and hepatitis B. Pregnant women generally had significantly lower immunoglobulin levels in comparison with non-pregnant women. In acute NANB hepatitis during pregnancy, serum IgG and IgM levels were lower and higher, respectively, compared with those in non-pregnant women and pregnant women with acute hepatitis B. It is suggested that an immune suppression during pregnancy might be responsible for increased susceptibility to acute NANB viral hepatitis, which, by itself, seems to induce only a transient acute phase IgM response.