Hydrazine (N2H4), which has been categorized as a weak carcinogen, is a chemical with the one of the largest production rates in Japan. We have investigated the effects of acetylation phenotypes on the metabolism of hydrazine. Genotypes of N-acetyl transferases, NAT2*, were determined using polymerase chain reaction for 297 male workers. Biological and exposure monitoring were also conducted. The rapid and intermediate acetylators accounted for 45% each, and the slow acetylators accounted for 10%. Biological half-lives were significantly different among the three acetylation phenotypes (analysis of variance, P < 0.05): 3.94 ± 1.70 hours for slow acetylators, 2.25 ± 0.37 hours for intermediate acetylators, and 1.86 ± 0.67 hours for rapid acetylators. Among Japanese, rapid and intermediate acetylators are the major phenotypes, which is in sharp contrast with those among Caucasians. We conclude that biological monitoring should take genetic factors, which may vary dramatically among different populations, into account.