diff --git a/spec/workspace-spec.coffee b/spec/workspace-spec.coffee
index 636755346..9a99d3ed2 100644
--- a/spec/workspace-spec.coffee
+++ b/spec/workspace-spec.coffee
@@ -224,7 +224,7 @@ describe "Workspace", ->
               expect(workspace.paneContainer.root.children[0]).toBe pane1
               expect(workspace.paneContainer.root.children[1]).toBe pane4
 
-    describe "when the file is large (over 2mb)", ->
+    describe "when the file is over 2MB", ->
       it "opens the editor with largeFileMode: true", ->
         spyOn(fs, 'getSizeSync').andReturn 2 * 1048577 # 2MB
 
@@ -235,6 +235,30 @@ describe "Workspace", ->
         runs ->
           expect(editor.displayBuffer.largeFileMode).toBe true
 
+    describe "when the file is over 20MB", ->
+      it "prompts the user to make sure they want to open a file this big", ->
+        spyOn(fs, 'getSizeSync').andReturn 20 * 1048577 # 20MB
+        spyOn(atom, 'confirm').andCallFake -> selectedButtonIndex
+        selectedButtonIndex = 1 # cancel
+
+        editor = null
+        waitsForPromise ->
+          workspace.open('sample.js').then (e) -> editor = e
+
+        runs ->
+          expect(editor).toBeUndefined()
+          expect(atom.confirm).toHaveBeenCalled()
+
+          atom.confirm.reset()
+          selectedButtonIndex = 0 # open the file
+
+        waitsForPromise ->
+          workspace.open('sample.js').then (e) -> editor = e
+
+        runs ->
+          expect(atom.confirm).toHaveBeenCalled()
+          expect(editor.displayBuffer.largeFileMode).toBe true
+
     describe "when passed a path that matches a custom opener", ->
       it "returns the resource returned by the custom opener", ->
         fooOpener = (pathToOpen, options) -> {foo: pathToOpen, options} if pathToOpen?.match(/\.foo/)
diff --git a/src/project.coffee b/src/project.coffee
index c10ce4a3b..b21e74076 100644
--- a/src/project.coffee
+++ b/src/project.coffee
@@ -323,8 +323,22 @@ class Project extends Model
         # allow ENOENT errors to create an editor for paths that dont exist
         throw error unless error.code is 'ENOENT'
 
-    @bufferForPath(filePath).then (buffer) =>
-      @buildEditorForBuffer(buffer, options)
+    absoluteFilePath = @resolvePath(filePath)
+
+    fileSize = fs.getSizeSync(absoluteFilePath)
+
+    if fileSize >= 20 * 1048576 # 20MB
+      choice = atom.confirm
+        message: 'Atom will be unresponsive during the loading of very large files.'
+        detailedMessage: "Do you still want to load this file?"
+        buttons: ["Proceed", "Cancel"]
+      if choice is 1
+        error = new Error
+        error.code = 'CANCELLED'
+        throw error
+
+    @bufferForPath(absoluteFilePath).then (buffer) =>
+      @buildEditorForBuffer(buffer, _.extend({fileSize}, options))
 
   # Retrieves all the {TextBuffer}s in the project; that is, the
   # buffers for all open files.
@@ -354,8 +368,7 @@ class Project extends Model
   # * `filePath` A {String} representing a path. If `null`, an "Untitled" buffer is created.
   #
   # Returns a promise that resolves to the {TextBuffer}.
-  bufferForPath: (filePath) ->
-    absoluteFilePath = @resolvePath(filePath)
+  bufferForPath: (absoluteFilePath) ->
     existingBuffer = @findBufferForPath(absoluteFilePath) if absoluteFilePath
     Q(existingBuffer ? @buildBuffer(absoluteFilePath))
 
@@ -405,7 +418,7 @@ class Project extends Model
     buffer?.destroy()
 
   buildEditorForBuffer: (buffer, editorOptions) ->
-    largeFileMode = fs.getSizeSync(buffer.getPath()) >= 2 * 1048576 # 2MB
+    largeFileMode = editorOptions.fileSize >= 2 * 1048576 # 2MB
     editor = new TextEditor(_.extend({buffer, largeFileMode, registerEditor: true}, editorOptions))
     editor
 
diff --git a/src/workspace.coffee b/src/workspace.coffee
index afc197792..564152df6 100644
--- a/src/workspace.coffee
+++ b/src/workspace.coffee
@@ -437,15 +437,16 @@ class Workspace extends Model
       item ?= atom.project.open(uri, options)
     catch error
       switch error.code
-        when 'EFILETOOLARGE'
-          atom.notifications.addWarning("#{error.message} Large file support is being tracked at [atom/atom#307](https://github.com/atom/atom/issues/307).")
+        when 'CANCELLED'
+          return Q()
         when 'EACCES'
           atom.notifications.addWarning("Permission denied '#{error.path}'")
+          return Q()
         when 'EPERM', 'EBUSY'
           atom.notifications.addWarning("Unable to open '#{error.path}'", detail: error.message)
+          return Q()
         else
           throw error
-      return Q()
 
     Q(item)
       .then (item) =>