In order to know the etiology and natural history of acute viral hepatitis in intravenous drugs abusers, a panel of 110 heroin addicts, suffering acute hepatitis, is studied. The B virus, by itself or in conjunction with the delta virus was responsible for 63 cases. 35 patients suffered of NANB hepatitis and only two patients were diagnosed of hepatitis A. Delta infection was detected in 38 of the 77 cases of HBsAg-positive acute hepatitis. All the hepatitis B patients had serum conversion within the first 6 months of the acute episode. All the patients suffering acute delta hepatitis and 75% of those suffering non-A, non-B hepatitis, developed chronic hepatic disease. We conclude that in our environment, the B virus is the main responsible for acute hepatitis in heroin addicts. Delta infection is detected in almost half of the heroin addicts with HBsAg-positive acute hepatitis. The acute hepatitis caused by B virus, or by B and delta viruses simultaneously, have a satisfactory evolution, thus, almost all the cases are cured within 6 months. On the contrary, hepatitis caused by NANB or delta viruses usually progresses to chronic hepatic disease.