A homozygous gene deletion of the glutathione S-transferase M1 (GSTM1) locus of genomic DNA from blood spots was studied by the polymerase chain reaction in a group of French heavy smokers (n = 361), which included patients with severe chronic bronchitis (SCB; n = 87), moderate chronic bronchitis (MCB: n = 102) and hard smokers (HS) with no permanent clinical symptoms of chronic bronchitis (n = 172). The GSTM1 0/0 genotype was found in 71.3% and 65.7% of cases in SCB and MCB, respectively, compared with only 47.1% in the control HS group (P = 0.0002). This latter figure (47.1%) is consistent with the average GSTM1 deletion frequency in French Caucasians. Moreover, the results showed a significant difference in the distribution of the GSTM1 0/0 genotype for both the SCB and MCB groups against the control HS group, according to gender (SCB: P = 0.001; MCB: P = 0.005), age (SCB: P = 0.0001; MCB: P = 0.005) and smoking history (SCB: P = 0.0001; MCB: P = 0.005). Thus, individuals homozygous for the GSTM1 gene deletion, especially in the under-41 age group (SCB: P = 0.001; MSB: P = 0.04) with an average smoking history of 16-30 pack-years (SCB: P = 0.002; MSB: P = 0.01) are more prone to chronic lung diseases, such as SCB and MCB, than are GSTM1 +/+ or 0/+ subjects. Population screening of young people for the identification of GSTM1 0/0 subjects, with special emphasis on smoking habits, might be useful (1) for the early detection of individuals at high risk of lung complications caused by environmental toxins and pollutants and (2) in clinical practice, in order to prevent the development of chronic bronchitis, which is a common disease.