ABSTRACT—The effect of cianidanol on the HBeAg/anti‐HBe system in 338 patients with HBeAg‐positive chronic hepatitis was studied in a double‐blind, randomised, placebo‐controlled, multicentre clinical trial. 174 patients received cianidanol in a daily dose of 1.5 g for 2 weeks, followed by 2.25 g for a further 14 weeks. 164 patients received a placebo for the same period: patients were followed up for a further 8 weeks. HBeAg and anti‐HBe antibody titers were measured by R.I.A. at 4‐week intervals and the results were expressed as a “cut‐off index” and “inhibition percent”, respectively. Liver function tests were also monitored at the same intervals. The HBeAg titer decreased by at least 50% in 44 of 144 cases treated with cianidanol (21 of 140 cases treated with placebo). The difference was significant (p<0.01). The HBeAg disappeared in 16 of the cianidanol cases and four of the placebo (p<0.05) and a seroconversion was observed in six cianidanol patients and three placebo patients. The mean HBeAg titer in the cianidanol group was significantly lower than that in the placebo group at the end of the 16 weeks of therapy (p<0.05). The patients whose HBeAg titer was lowered were largely those with chronic active hepatitis and had higher initial values of SGPT, SGOT and γ‐globulin than the patients whose HBeAg titers remained unchanged. The mean values for these liver function tests also fell significantly in the former sub‐group. The drug was well tolerated, the only notable side effect being a transient febrile reaction in 13 patients. It is concluded that cianidanol is a useful and well‐tolerated drug for improving the HBeAg/anti‐HBe system in patients with HBeAg