Both case-control and cohort studies were undertaken from July 1, 1969 to June 30, 1983 to ascertain whether exposure to chloroprene increases the risk of cancer. Fifty-five cases of cancer deaths were verified, 16 of which had histories of exposure to chloroprene ranging from 3 to 23 years (median 11 years) with a latent period of 8-27 years, except for one case of 3 years (median 12.5 years). Fifty-four pairs were obtained by matching the cancer deaths to noncancer deaths in accordance with strict requirements. The odds ratio for the paired data was 13, X2 = 8.64, P less than 0.005. The average age at death from cancer of workers exposed to chloroprene was 12.7 years younger than that of unexposed workers, t' = 2.98, P less than 0.001. The total cohort consisted of 1213 persons, among whom 149 (11.6%) had histories of exposure for over 25 years, 381 (31.5%) for over 20 years, and 852 (70.2%) for over 15 years. The SMR for the total cohort was 2.38 (P less than 0.01), and all SMRs for the high-exposure occupations were of significance (P less than 0.05 or P less than 0.01), in contrast to those of the low-exposure grups whose SMRs were low or zero. Thus, a dose-response relationship existed. Among the high-exposure occupations, maintenance mechanics seem to have the highest risk of cancers, and SMRs for liver, lung, and lymphatic cancers were significant in this group. These results suggested that chloroprene exposure increases the risk of developing cancer.