To assess the risk of lung cancer and mesothelioma after environmental exposure to crocidolite for 20-30 years, a retrospective cohort study was carried out in farmers who had been exposed to crocidolite in environment. 1610 subjects were followed up for 9 years (Jan. 1, 1987 Dec. 31, 1995). The control group consisted of 7646 farmers who resided in the noncrocidolite pollution rural area in the same province. The results showed that the annual mortality rate was 43.75 per 100,000 population for lung cancer, and 36.46 per 100,000 for mesothelioma. Significantly high risks of lung cancer (RR 5.67) and mesothelioma (RR 182.3) were noted. These results demonstrate a strong causal association between lung cancer, mesothelioma and exposure to crocidolite.