In the case of Castaneda v. Partida, the U.S. Supreme Court ruled in favor of Partida, holding that there was a discriminatory effect on Mexican-Americans in Hidalgo County's grand jury selection process which violated their Fourteenth Amendment rights to equal protection under law. The court found that over an 11-year period, only 39% of those selected for grand juries were Mexican-American despite making up nearly 80% of the county population who were eligible for jury service. This significant disparity suggested discrimination and could not be explained by chance alone according to statistical evidence presented during trial proceedings. Therefore, it was concluded that this system did not provide fair representation and thus breached constitutional protections against racial discrimination within judicial processes.

In the dissenting opinion for Castaneda v. Partida, Justice William Rehnquist disagreed with the majority's interpretation of statistical evidence as proof of discrimination in jury selection. He argued that a mere disparity between population and representation does not necessarily indicate intentional bias or prejudice. Furthermore, he contended that it was inappropriate to apply standards from voting rights cases to this context because they involve different legal issues and societal impacts. In his view, the Court should have required more concrete evidence demonstrating discriminatory intent rather than relying on statistics alone.