A homozygous gene deletion at the glutathione S-transferase M1 (GSTM1) locus of genomic DNA from blood spots was studied by PCR in the group of Slavic populations from the north-western and central-eastern regions of European Russia and in patients with lung cancer (LC), other tumors (OT), endometriosis (E), alcoholic cirrhosis (AC), cystic fibrosis (CF) and chronic bronchitis (CB). The frequencies of the GSTM1 0/0 genotype were 38.8% and 67.5% for both population groups, respectively. The proportion of the GSTM1 gene deletion genotype was estimated as significantly increased in LC (81%), OT (65%), E (81%), AC (77.3%), and in CB (73.6%) patients with symptoms of CB confirmed by X-ray but not in CB patients without X-ray evidence of disease (40.9%). A definite preponderance of GSTM1-0 homozygotes (51.1%) has been registered in CF patients of the pancreatic sufficient group with clear-cut pulmonological manifestations but not in those of the pancreatic insufficient group with predominantly intestinal or mixed clinical symptoms (41.2% and 37.5%, respectively). Earlier clinical manifestations and death before the age of 5 years are typical for GSTM1-deleted CF patients. These data support the notion that GSTM1 deletion should be considered as a convenient genetic marker for the early detection of groups at higher risk of many diseases caused by environmental and genetic factors, where manifestation depends on the lack of detoxification. High levels of GSTM1 0/0 genotypes in E patients favor the substantial contribution of certain environmental toxins in the pathogenesis of this widespread disease.