In 1983, intervention of precancerous lesion of esophagus was undertaken in the high risk area of esophageal cancer, Heshun Village, Linxian County. It had been expected that cancerous degeneration rate of esophageal dysplasia should be reduced by 50% so as the prevention of esophageal cancer could become possible. 6758 subjects of the general population aging from 40 to 65 were examined by esophageal exfoliative cytology, 1729 had marked dysplasia and 2411 had mild dysplasia of esophageal epithelium. Those with marked dysplasia were randomized into 3 groups to take their respective medication: antitumor B (Chinese herbs); retinamide (4-Ethoxycarbophenylretinamide) and placebo. The subjects with mild dysplasia were divided randomly into 2 groups for treatment by riboflavin and placebo. 95% of the subjects had taken 90% or more of the total medication for 3 years, at the end of which they were reexamined by esophageal exfoliative cytology. The reexamination rate was 94.1%. The incidence of esophageal cancer in the antitumor B group (3.9%) was reduced by 53% as compared with that of the placebo group (8.3%). This difference had statistical significant (means 2 = 7.672, P less than 0.05). The incidence of esophageal cancer in retinamide and riboflavin groups were reduced by 33.7% and 19% as compared with those of the control groups. The regression rate of dysplasia in the treatment groups were increased than that of the control groups. The above results showed that our hypothesis about the secondary prevention of esophageal cancer is correct. The intervention of precancerous lesion of the esophagus is effective in the prevention of esophageal cancer.(ABSTRACT TRUNCATED AT 250 WORDS)