The aims of our meta-analysis were to examine the pattern and gender's influence on body mass index (BMI) - pneumonia relationship. Published studies were searched from PubMed, Web of Science, Cochrane Library databases using keywords of pneumonia, BMI and epidemiologic studies. Random-effects analysis was applied to estimate pooled effect sizes from individual studies. The Cochrane Q-test and index of heterogeneity (I2 ) were used to evaluate heterogeneity, and Egger's test was used to evaluate publication bias. Random-effects meta-regression was applied to examine the pattern and gender's influence on BMI-pneumonia relationship. A total of 1,531 studies were initially identified, and 25 studies finally were included. The pooled relative risk (RR) and meta-regression model revealed a J-shaped relationship between BMI and risk of community-acquired pneumonia (underweight, RR 1.8, 95% confidence interval [CI], 1.4-2.2, P?<?0.01; overweight, 0.89, 95%CI, 0.8-1.03, P, 0.1; obesity, 1.03, 95% CI, 0.8-1.3, p. 8) and U-shaped relationship between BMI and risk of influenza-related pneumonia (underweight, RR 1.9, 95% CI, 1.2-3, P?<?0.01; overweight, 0.89, 95% CI, 0.79-0.99, P, 0.03; obesity, 1.3, 95% CI, 1.05-1.63, p. 2; morbidity obesity, 4.6, 95% CI, 2.2-9.8, P?<?0.01); whereas, no difference in risk of nosocomial pneumonia was found across the BMI groups. Gender difference did not make significant contribution in modifying BMI-pneumonia risk relationship.