Objective To evaluate the relationship between body mass index (BMI) and malignant lymphoma by means of Meta-analysis. Methods Such databases as Web of Science, PubMed, EBbase, CNKI, Wanfang, VIP and CBM were searched from the date of their establishment to April 2011 to collect the case control studies on the relationship between BMI and malignant lymphoma. Two researchers independently selected studies, extracted data and assessed the quality according to the inclusive and exclusive criteria, and then conducted Meta-analyses by using RevMan5.0 software for heterogeneity test and pooled OR calculation. Results Seven case control studies involving 8416 malignant lymphoma patients and 14760 other patients were included. The quality of all studies scored 4, indicating reliable quality. Meta-analyses of the low BMI, overweight and obesity population were OR=0.8, 95%CI 0.79 to 0.95, P=0.003; OR=1.04, 95%CI 0.98 to 1.11, P=0.16; and OR=1.22 95%CI 1.04 to 1.43, P=0.01, respectively. The stratified Meta-analysis on histological subtypes showed that obesity was associated with a significantly increased risk of diffuse large B cell lymphoma (OR=1.33 95%CI 1.18 to 1.50, Plt;0.000 01), but was not associated with the follicular lymphoma or small lymphocytic lymphoma/chronic lymphocytic leukemia. Conclusion These findings demonstrate that low BMI is associated with the decrease of malignant lymphoma, and obesity is an increasing risk of malignant lymphoma, especially, the diffuse large B cell lymphoma.