TGF beta 1 is an important cytokine acts as an antiinflammatory agent, and inhibits B cell proliferation. In patients with chronic lymphocytic leukemia (CLL), serum level of TGF beta 1 are found elevated. Presence of G allele at position +915 in TGF beta 1 gene results in arginine synthesis which is associated with higher expression of TGF beta 1. We investigated the association of TGF beta 1 +915G>C polymorphism with predisposition, clinical characteristics and laboratory findings of CLL. 50 CLL patients and 50 healthy controls were included in this study. Genotypes were determined by PCR-RFLP method. We couldn't find statistically significant differences between patient and control groups in terms of genotype distributions and allele frequencies. However, GC genotype frequency was slightly higher in CLL patients than healthy controls. Furthermore, TGF beta 1 +915GC genotype was found associated with trisomy 12 (p = 0.007). Further studies are needed to clarify exact role of TGF beta 1+915G>C polymorphism in patients with CLL.