Occupants＇　window　opening　behavior　influences　the　performance　of　buildings　significantly.　Good　window　use　can　provide　a　comfortable　indoor　environment　with　a　minimum　energy　use,　while　improper　window　use　may　result　in　bad　indoor　environment　and　may　also　waste　great　energy.　This　paper　identifies　improper　window　uses　in　an　existing　building　in　Beijing,　China,　based　on　a　year-long　longitudinal　monitoring　of　occupants＇　window　opening　behavior,　together　with　important　indoor　and　outdoor　environmental　variables.　The　findings　from　this　study　provide　evidence　that　occupants＇　window　opening　behavior　in　real　buildings　is　required　to　be　improved　to　promote　the　energy　efficiency　of　the　building.　(C)　2016　The　Authors.　Published　by　Elsevier　Ltd.