The occurrence of the delta (delta) agent was analyzed in 89 patients with acute hepatitis B infection during 1976-1979 in Gothenburg, Sweden, and in 46 patients (16 drug addicts) with chronic HBsAg-positive liver disease. Four of the patients with acute hepatitis B had transiently detectable anti-delta antibodies in serum. At least three of these four cases were associated with intravenous drug abuse. Eleven of the HBsAg carriers (24%) were anti-delta-positive, and all of them were drug addicts. One of the drug addicts transmitted hepatitis B infection without detectable anti-delta in serum to two other non-addicts via parenteral routes. Apparently, in Sweden today delta-infection is mostly restricted to drug addicts and seldom found in other groups of hepatitis B patients.