Polymorphisms associated with genes coding for glutathione S-transferase enzymes are known to influence metabolism of different carcinogens and have been associated with incidence of various types of cancer. We have determined the GST M1 and GST T1 'null' genotype frequency in 81 patients with chronic myeloid leukaemia (CML) and 123 racially and geographically matched control individuals by multiplex polymerase chain reaction (PCR). GST M1 null genotype frequencies in CML and controls were 28.4% and 27.7%, respectively. GST T1 null genotype frequencies in CML and controls were 19.8% and 7.3%, respectively. The GST T1 null genotype frequency in CML patients is significantly different from that in controls (odds ratio (OR) 3.12, 95% confidence interval (CI) 1.3-7.45, P=0.008).