Abstract We argue that administrative agencies should base their decisions on Cost-Benefit Analyses for two reasons: first the well known ex ante effect that CBA make public decisions more rational and second the little discussed ex post effect that the agency’s CBA provides valuable information to judicial or political bodies controlling the agency. The ex post purpose of CBA implies (judicial review of administrative decisions) or requires (political review) the public availability of the CBA. A conflict between this public availability and the protection of trade secrets arises, if the costs or benefits of a decision depend on information which are trade secrets. As a solution to this conflict, we propose a non-public in camera court procedure which leaves little discretion to the agency.