With the finding of HBV and HAV, it is possible at the present time to recognize according to its etiology three types of viral hepatitis: type A, Type B and type non-A-non-B. In this paper we have proved that a low socioeconomic status plays a very important role in the spreading of this disease. In a community with a low socioeconomic status, of forty children attending the fourth grade of a primary school, 97 percent were found to have anti-HA: while in a similar group of children with a higher socioeconomic background, we found only 40 per cent of positive cases. A positive sero convertion to anti-HA was found in 90 per cent of the 73 children with acute hepatitis; the remaining 10 per cent were non-A-non-B. A group of 61 children admitted to the L. Calvo Mackenna Children's Hospital with acute or chronic non hepatic disease were studied for anti-HA. We found 77% positive cases in the group of infants under 4 months old; in older infants, a low incidence of anti-HA was observed, but a notorious increase of positive cases was seen after the age of two years, reaching 100 percent of positive cases in children above four years of age. In ten patients studied with prolonged hepatitis, five of them could be possibly classified as having a non-A-non-B hepatitis.