The aim of this article is to study the relationship between GSTM1 polymorphism and colon cancer and to compare the chromosomal breakage induced by mutagen in a colon cancer group and healthy controls. Using PCR to identify the GSTM1 genotype, we found the frequency of GSTM1- in colon cancer (n = 19) and control group (n = 23) was 36.8% and 26.1%, respectively (p > 0.05, chi 2-test). The bleomycin-induced chromosomal breakage (break/cell) in the patient group was 0.75 +/- 0.29, and in the control group 0.42 +/- (0.24) (p < 0.05, t-test). The percentage of mutagen sensitivity (b/c > 0.8) in the patient group (68%) was 4 times as high as that in the control group (20%). The mutagen hypersensitivity (b/c > 1.0) in the patient group (47%) was 5 times as high as that in the control group (12%). The odds radio was 6.6.