Little is known about the pathologic characteristics of viral hepatitis A in humans. The authors compared the histologic features in liver biopsy specimens taken within 30 days of the onset of illness from 15 patients with hepatitis A and 14 patients with acute hepatitis B. In both hepatitis A and B, liver cell damage and necrosis were diffusely located, with accentuation in the centrilobular and midzonal areas in which ballooning degeneration and variation in cytoplasmic staining quality were observed frequently. One case of epidemic hepatitis A showed prominent periportal liver cell necrosis with inconspicuous centrilobular liver cell alterations. Kupffer cell mobilization was mild in hepatitis A, but more striking in hepatitis B. The portal inflammation was more pronounced and rich in plasma cells in hepatitis A than in hepatitis B. In summary, there were no major differences in the pathologic features of acute hepatitis A and B as sampled within 30 days of the onset of illness.