This essay aims to conduct a comparative inquiry into the doctrine of unilateral mistake in U.S. and Italian contract law, adopting the methodological lens of law and economics in order both to examine the effects that the various rules produce on the conduct of contracting parties and to express an assessment, in terms of economic efficiency, of the two legal systems. The analysis unfolds, first, through a comparison between the rigid taxonomy of art. 1429 of the Italian Civil Code and the elastic standard of the basic assumption of American origin, showing that this divergence is reflected in the degree of predictability of judicial decisions and in the diligence burdens placed on the parties. The article then highlights how the architecture of remedies under Italian law-thanks to the joint operation of the restrictive filter of the determining mistake and of the corrective mechanism of art. 1432 of the Civil Code-is better suited to preserving efficient exchanges than the U.S. system. The article goes on to examine the rule of recognizability of the mistake, adopted by both legal systems, and reads it in the light of the doctrine of last clear chance. With respect to the duty to inform the counterparty, the article highlights the operational fragility of the safe harbor provided under American law to the benefit of the informed buyer. Finally, an attempt is made to propose an economic justification for the Italian rule that establishes voidability for the recognized mistake without providing zones of immunity for the informed party. This rule is interpreted as a possible response to the no-trade problem: the buyer's silence comes, as a rule, to constitute a credible signal of the absence of undisclosed private information, with the result that the rational attitude of suspicion that often blocks bilateral negotiations is mitigated.