diff --git a/lib/pessimize/gemspec.rb b/lib/pessimize/gemspec.rb
new file mode 100644
index 0000000..590ee36
--- /dev/null
+++ b/lib/pessimize/gemspec.rb
@@ -0,0 +1,27 @@
+require 'pessimize/gemfile'
+
+module Pessimize
+  class Gemspec < Gemfile
+  protected
+
+    def parse_gems_from_tokens!
+      enum = tokens.each_with_index
+
+      loop do
+        (tok, i) = enum.next
+
+        if tok[1] == :on_ident && (tok[2] == "add_dependency" || tok[2] == "add_development_dependency")
+          gem_toks = []
+          until all_gem_tokens_collected?(tok[0], enum.peek[0])
+            (tok, j) = enum.next
+            gem_toks << tok
+          end
+
+          self.gems << Pessimize::Gem.new(gem_toks)
+          self.gem_token_map << [i + 1, j]
+        end
+      end
+    rescue StopIteration
+    end
+  end
+end
diff --git a/lib/pessimize/pessimizer.rb b/lib/pessimize/pessimizer.rb
index 31eb5b9..b3bcdee 100644
--- a/lib/pessimize/pessimizer.rb
+++ b/lib/pessimize/pessimizer.rb
@@ -1,4 +1,5 @@
 require 'pessimize/gemfile'
+require 'pessimize/gemspec'
 require 'pessimize/gemfile_lock_version_parser'
 require 'pessimize/version_mapper'
 
@@ -13,7 +14,8 @@ def initialize(file_manager, options)
     def run
       collect_gems_and_versions
       update_gem_versions
-      write_new_gemfile
+      puts gemfile.to_s
+      # write_new_gemfile
       puts "~> written #{gemfile.gems.length} gems to Gemfile, constrained to #{options[:version_constraint]} version updates\n\n"
     end
 
@@ -24,8 +26,16 @@ def sep(num = 1)
       "\n" * num
     end
 
+    def file_to_pessimize
+      if options[:gemspec]
+        Gemspec.new(File.read(options[:gemspec]))
+      else
+        Gemfile.new(file_manager.gemfile_contents)
+      end
+    end
+
     def collect_gems_and_versions
-      self.gemfile = Gemfile.new(file_manager.gemfile_contents)
+      self.gemfile = file_to_pessimize
       lock_parser.call File.open(file_manager.gemfile_lock)
     end
 
diff --git a/lib/pessimize/shell.rb b/lib/pessimize/shell.rb
index 0aeac3f..b5e49d4 100644
--- a/lib/pessimize/shell.rb
+++ b/lib/pessimize/shell.rb
@@ -37,6 +37,7 @@ def cli_options
 
         opt :version_constraint, "Version constraint ('minor' or 'patch')", default: 'minor', type: :string, short: 'c'
         opt :backup, "Backup existing Gemfile and Gemfile.lock", default: true, type: :boolean, short: 'b'
+        opt :gemspec, "Pessimize gemspec", type: String, short: 's'
       end
     end