Objective To investigate the changes in serum lactic acid level and its relationship with prognosis in the elderly patients with severe pulmonary infection in order to provide a basis for the diagnosis, treatment and monitoring of the disease. Methods A total of 76 hospitalized elderly patients with pulmonary infection between October 2011 to May 2013 were enrolled in this study, and another 35 healthy matched individuals chosen randomly from those taking physical examination at the same time period served as normal controls. Their serum levels of lactic acid were detected by lactate oxidase method, and the results were compared and analyzed. Results The serum level of lactic acid was significantly higher in pulmonary infection group than in control group [(3.68±1.43) vs (1.59±0.30)mmol/L, P＜0.05]. The percentage of the elderly with abnormal lactic acid was also higher in pulmonary infection patients than in controls (85.53% vs 5.70%, P＜0.05). The serum level was significantly increased in the patients ＞70 years old than in those between 60 to 70 years old [(4.64±2.05) vs (3.06±0.87)mmol/L, P＜0.05]. The level was also obviously higher in dead patients than in survival patients [(5.31±0.88) vs (3.01±1.02)mmol/L, P＜0.05]. Conclusion The serum level of lactic acid is significantly higher in the elderly patients with severe pulmonary infection. With the increase in the age, the serum level of lactic acid is elevated significantly, which is often associated with the disease prognosis. Serum level of lactic acid might be used as a comprehensive index of disease monitoring and prognosis in elderly patients with severe pulmonary infection.