Based on observations of a limited number of patients, delta (δ) infection has been reported to be infrequent in Taiwan. To further evaluate the role of δ‐infection in patients with hepatitis B virus (HBV) infection, serum samples of 493 subjects with acute and chronic HBV infections collected in 1976–85 were studied for anti‐δ by radioimmunoassay. Intrahepatic δ‐antigen was also studied by immunofluorescence in 12 anti‐δ‐positive patients. The overall prevalence of δ‐infection was 4.7%, consistent with previous studies. δ‐Infection had an even yearly distribution in the last decade. However, there were four groups with significantly higher frequencies: (i) 24% of 41 anti‐HBe‐positive patients with chronic active hepatitis (CAH); (ii) 21% of 14 HBsAg carriers with prominent lobular hepatitis; (iii) two of three HBsAg carriers with intravenous drug abuse; and (iv) two of seven with fulminant hepatitis. On the other hand, δ‐infection was uncommon in patients with chronic persistent hepatitis, cirrhosis, hepatocellular carcinoma, classical type B hepatitis, submassive necrosis and asymptomatic HBsAg carriers. δ‐Antigen was found in only two patients with CAH; one progressed to cirrhosis, and the other had disease regression on follow‐up. Overall, at least half of the δ‐superinfected HBsAg‐positive patients had a non‐progressive course on follow‐up.It was concluded that δ‐agent was introduced to Taiwan before 1976. Although it has played a role in some clinical settings of HBV infections, it is generally infrequent in Taiwan. The δ‐superinfection apparent in half the patien