The aetiology of sporadic acute hepatitis in Taiwan was studied in a consecutive series of 273 patients, 209 males and 64 females, aged from less than 1 to 73 years. Only eight patients (2.9%) had acute hepatitis A, 36 (13.2%) acute hepatitis B and 57 (20.9%) had acute non-A, non-B hepatitis (NANB). The remaining 172 patients (63.0%) were HBsAg positive but anti-HBc IgM negative and were classified as acute hepatitis in chronic HBsAg carriers. Age-specific analysis revealed that acute hepatitis types A and B accounted for 35% and 47%, respectively of acute hepatitis in patients below age 10, but their prevalence decreased dramatically with increasing age and remained quite low in patients over age 40. In contrast, acute hepatitis NANB occurred predominantly in the older patients. The prevalence of acute hepatitis NANB was 11 to 15% in patients below age 40, but increased gradually with increasing age thereafter. Of the patients over age 60, 58% had acute hepatitis NANB. Acute hepatitis in chronic HBsAg carriers accounted for 59 to 76% of acute hepatitis episodes in patients over age 10 and below age 60. Sex-specific analysis revealed that the prevalence of acute hepatitis A or B showed no significant sex difference, whereas acute hepatitis in chronic HBsAg carriers occurred predominantly in males and acute hepatitis NANB occurred predominantly in females. Of the acute hepatitis episodes in chronic HBsAg carriers about one-third was due to hepatitis Delta virus superinfection, while the remaining two-thirds were most likely acute exacerbations of the underlying hepatitis B virus infection and a few were due to non-A, non-B hepatitis superinfection.