Background and Objectives: Sevoflurane has opposing effects on cancer progression, depending on its concentration and the cancer type. This study investigated the effects of sevoflurane on the proliferation of A549 lung cancer cells. Materials and Methods: In vitro, the number of A549 cells exposed to different concentrations of sevoflurane was counted. The size and weight of tumors from a xenograft mouse model exposed to air or sevoflurane were measured in vivo experiments. Additionally, hematoxylin and eosin staining and immunohistochemical detection of Ki-67 in the harvested tumor tissues were performed. Results: A total of 72 culture dishes were used and 24 dishes were assigned to each group: Air group; 2% Sevo group (air + 2% sevoflurane); and 4% Sevo group (air + 4% sevoflurane). The number of A549 cells in the 2% Sevo group was less than that in the Air and 4% Sevo groups (Air: 7.9 ± 0.5; 0.5, 2% Sevo: 6.8 ± 0.4, 4% Sevo: 8.1 ± 0.3; p = 0.000). The tumor size was not significantly different between the two groups (Air: 1.5 ± 0.7, 2% Sevo: 2.4 ± 1.9; p = 0.380). Conclusions: The in vitro data showed that sevoflurane inhibited the proliferation of A549 lung cancer cells in a concentration-specific manner. However, the in vivo data showed no correlation between sevoflurane exposure and A549 cell proliferation. Thus, further research is required to understand fully the effects of sevoflurane on cancer progression and to reconcile differences between the in vitro and in vivo experimental results.