CONTEXT HBV infection is endemic in Brazil and acute HBV infection is still a common disease. OBJECTIVE To analyze incidence, risk factors and evolution of acute HBV infection. SETTING University Hospital. PATIENTS 357 patients with acute HBV infection, comparing two periods: 1985-1989 vs. 1990-1994. RESULTS The overall incidence declined from 50 new cases/year in 1985-89 (30% of all cases) to 25 new cases/year in 1990-94 (8% of all cases). Transmission among male homosexuals (3.9% of cases in 85-89) declined to 1.3% in 90-94 (p > 0.05). Amongst health care workers (HCW) it declined from 8.2% to 2.0% (p = 0.02). Conversely, heterosexual transmission increased from 4.8% to 10.1% (p = 0.06). Chronification of HBV infection following the acute episode was observed in 1.7%. Fulminant hepatic failure was seen in 3.4%. However, 27.2% of patients were lost to follow-up before normalization of the biochemical tests. CONCLUSIONS The different patterns of risk factors observed is probably related to measures for preventing AIDS and to HCW vaccination programs. Chronification following acute episodes was not a common event.