An important administrative law doctrine developed by the lower federal courts, called remand without vacatur, rests on a mistaken premise. Courts that embrace the doctrine maintain that when they find that a federal agency regulation is unlawful, they have discretion to remand the regulation without vacating it. The remand gives the regulatory agency an opportunity to correct the flaws that render the regulation unlawful. When a regulation is remanded but not vacated, the courts assume, the regulation binds regulated parties despite its illegality. Unlawful regulations, however, are in general void ab initio, just as unconstitutional statutory rules are void ab initio. No affirmative judicial act is required to cause an unlawful regulation to become non-binding. In that respect, agency regulations are unlike lower-court decrees, which are binding when issued even if erroneous. Reviewing courts therefore do not have the option of allowing unlawful regulations to remain in effect, because unlawful regulations never go into effect. This article uncovers the implicit and undefended assumption of ab initio validity of unlawful regulations on which remand without vacatur rests, shows that the assumption is in general unsound, and lays out some of the implications of that conclusion.