The aim of this study was to detect the possible role of methylene tetrahydrofolate reductase gene polymorphism (MTHFR C677T) in the pathogenesis of lymphoid neoplasms and to investigate the influence of this polymorphism on methotrexate toxicity in adult ALL patients treated with methotrexate maintenance therapy. There was a statistically significant increase in the risk of non-Hodgkin lymphoma in patients with CT genotype (OR, 2.9; 95% CI, 1.3–6.3; P = 0.007) and combined CT + TT genotype (OR, 3.2; 95% CI, 1.5–6.6; P = 0.006). While no significant association was found between this polymorphism and ALL risk. The patients with ALL treated with methotrexate during maintenance therapy were observed for signs of toxicity. MTHFR 677C>T polymorphism (CT + TT) was significantly overrepresented among cases with hepatic toxicity (OR = 15.6; 95% CI, 2.6–81.3; P = 0.001). In addition, they were overrepresented among cases with mucositis, anemia, thrombocytopenia, and leukopenia. However, it did not reach statistical significance level. Further studies on larger number of subjects are necessary. Additional studies on the role of MTHFR gene polymorphism with environment (folate intake) interaction are needed to confirm the role of these genetic polymorphisms.