A previous study of 159 consecutive cases of HBsAg+ acute viral hepatitis, observed from the onset of disease and seen between May 1977 and May 1980, revealed a higher frequency of evolution to chronicity in the region of Naples than that reported for other areas. This suggested that environmental factors might influence the chronically rate. We investigated 125 HBsAg+ acute hepatitis cases for whom sera were available: 35 patients who remained HBsAg+ at the two-year control (28 chronic hepatitis and 7 healthy carriers) and 90 who recovered and seroconverted HBsAg---- anti-HBs during follow-up. At the onset of disease, all patients who cleared HBsAg showed IgM anti-HBc positivity and 48 were also HBeAg positive; no patient was anti-delta positive. On the contrary of the 28 chronic cases only 7 were IgM anti-HBc+ (6 under 13 years old), while 21 were IgM anti-HBc- but anti-delta+. This study demonstrates that not all HBsAg+ acute hepatitis cases are due to HBV and that in our area, superinfection by the delta agent is responsible for most cases of chronic HBsAg positive hepatitis.