SummaryTNR/11q#1 is a polymorphic trinucleotide (GCC)n repeat located within the minimal region of the 11q deletion in chronic lymphocytic leukemia (CLL). It was recently shown that certain alleles of this repeat are associated with a worse prognosis in CLL patients. To investigate the role of TNR/11q#1 variants as risk-modifying factors in leukemogenesis, we conducted a case-control study on 113 acute lymphotic leukemia (ALL) patients, 82 CLL patients and 146 healthy controls of Russian origin. Comparison of allele and genotype distributions in the control, ALL and CLL groups, performed by Fisher's exact test with two-sized P-value, showed significant decrease in the presence of the GCC 6 allele in the ALL and CLL groups compared to controls. Moreover, 'rare' alleles GCC 7-8 and GCC [13][14] were significantly overrepresented in the ALL group versus controls. We found that CLL risk genotypes were those with both alleles containing more than 6 GCC repeats (P = 0,0212, odds ratio = 1,68 (95% CI, 1 121. . . 2,531)). ALL risk genotypes include three allele combination variants: 1) both alleles containing more than 6 GCC repeats (P = 0,0019, odds ratio = 1,756 (95% CI 1,223. . .2,502)); 2) one of the alleles containing 7 or 8 repeats (P = 0,0155, odds ratio = 18,22 (95% CI 1,93. . .136.37)); 3) one of the alleles containing more than 12 repeats (P = 0,0209, Odds ratio = 2,599 (95% CI 1,161. . .5,815)). Association of certain alleles and genotypes of the TNR/11q#1 repeat with both acute and chronic lymphocytic leukemia suggests the presence of a cancer related gene, involved in a wide spectrum of neoplasia, in the vicinity of this repeat.