The 4th US FDA/Industry Workshop on Pharmacogenomics in Drug Development and Regulatory Decision Making, was held in MD, USA, on December 10-12, 2007. One of the breakout sessions of the workshop focused on the regulatory issues around codevelopment of drugs and companion diagnostics. This session used hypothetical case studies as focal points for discussion of current thought and critical issues for both industry and the FDA in this evolving field. The panel and the audience discussed the evolution of the FDA's thinking on the regulatory path for companion diagnostics since the release of the April 2005 draft Drug Test Codevelopment Concept Paper and the issues faced by industry in attempting codevelopment efforts. This session provided an opportunity to allow an exchange of ideas between the FDA and industry and to identify critical issues that need further discussion in this important and evolving field.