The Securities and Exchange Commission (SEC) v. National Securities, Inc., et al., 1968 is a landmark case that established the materiality standard in securities fraud cases. The SEC alleged that National Securities, Inc. had violated federal securities laws by failing to disclose certain information during a merger with another company. The Supreme Court held that the omission or misrepresentation of facts must be considered "material" for it to constitute fraud under Section 10(b) and Rule 10b-5 of the Securities Exchange Act of 1934. Materiality was defined as any fact which would have significantly altered the total mix of information available to an investor when deciding whether or not to invest in a security; essentially, if there is substantial likelihood that disclosure would have been viewed by reasonable investors as having significantly altered their investment decision.

In the dissenting opinion for SECURITIES AND EXCHANGE COMMISSION v. NATIONAL SECURITIES, INC., et al., Justice Harlan argued that the majority had overstepped its bounds by interpreting Section 14(a) of the Securities Exchange Act in a way that went beyond what Congress intended. He contended that this section was meant to prevent management from using deceptive practices to influence shareholder voting, not to provide shareholders with all material information about an upcoming vote. Furthermore, he disagreed with the majority's view that National Bank’s failure to disclose details about Arizona Corporation Commission's disapproval constituted a breach of fiduciary duty under state law. According to him, there was no clear evidence showing such violation and it should be left up for state courts' determination rather than federal courts'. Lastly, he expressed concern over potential negative impacts on corporate transactions due to increased legal uncertainties caused by this ruling.