diff --git a/Library/Homebrew/os/mac.rb b/Library/Homebrew/os/mac.rb
index c6fb9b72155aad35ca0343c60d2d3ab2bb7deea9..f9bf2e8786f91f5d880d9c74526774443815df63 100644
--- a/Library/Homebrew/os/mac.rb
+++ b/Library/Homebrew/os/mac.rb
@@ -24,7 +24,7 @@ module OS
     # This can be compared to numerics, strings, or symbols
     # using the standard Ruby Comparable methods.
     def full_version
-      @full_version ||= Version.new(ENV["HOMEBREW_MACOS_VERSION"].chomp)
+      @full_version ||= Version.new((ENV["HOMEBREW_MACOS_VERSION"] || ENV["HOMEBREW_OSX_VERSION"]).chomp)
     end
 
     def prerelease?