Regression bugs occur whenever software functionality that previously worked as desired stops working, or no longer works as expected. Code changes, such as bug fixes or new feature work, may result in a regression bug. Regression bugs are an annoying and painful phenomena in the software development process, requiring a great deal of effort to localize, effectively hindering team progress. In this paper we present Regression Detective, a method which assists the developer locating source code segments that caused a given regression bug. Unlike some of the existing tools, our approach doesn't require an automated test suite or executing past versions of the system. It is highly scalable to millions of loc systems. The developer, who has no prior knowledge of the code or the bug, reproduces the bug according to the steps described in the bug database. We evaluated our approach with bugs from leading open source projects (Eclipse, Tomcat, Ant). In over 90% of the cases, the developer only has to examine 10-20 lines of code in order to locate the bug, regardless of the code base size.