In order to verify the relative role of each single risk factor during a long period of observation, and to compare the frequency of risk factors in parenterally and non-parenterally transmitted acute viral hepatitis, we studied 1,251 patients admitted to our Department from 1971 to 1991. Acute hepatitis A cases were considered non-parenterally transmitted, whereas B, C, NANB and Delta hepatitis were grouped together as parenterally transmitted. The two groups were compared for age, sex and the following risk factors: surgical procedures, transfusion, dental procedures, intravenous drug addiction, infected partner, infected relative and hospital admission. There were 243 non-parenterally transmitted and 1,008 parenterally transmitted cases. In univariate analysis, mean age in the two groups was 20 and 37 years (p = 0.000001) for non-parenterally and parenterally transmitted cases respectively; mean ages of patients with different parenterally transmitted hepatitis (B, NANB, C, Delta) did not differ significantly (p = 0.35). The following risk factors were significantly more frequent in the parenterally transmitted hepatitis group: surgical procedure (odds ratio = 8.04, 95% confidence intervals: 3.75, 20.51), transfusion (OR = 18.79, 95% CI: 5.03, 157.72), dental procedures (OR = 2.19, 95% CI: 1.2, 4.06), drug addiction (OR: 11.02, 95% CI: 4.15, 41.34), and infected partner (OR = 17.61, CI: 3.02, 708.65). However, logistic regression showed the following factors as being significant: age (p = 0.00001), transfusion (OR = 3.35, 95% CI: 1.61, 6.94), dental procedures (OR = 1.61, 95% CI: 1.18, 2.2), drug addiction (OR = 4.88, 95% CI: 2.94, 8.1).(ABSTRACT TRUNCATED AT 250 WORDS)