Double PIK3CA mutations occur in cis on the same allele and result in increased PI3K activity, downstream signaling, cell proliferation, and tumor growth. Mechanistically, these mutations increase disruption of p110α binding to the inhibitory subunit p85α, relieving catalytic inhibition, while also increasing p110α membrane lipid binding. Importantly, double PIK3CA mutations predict increased sensitivity to PI3Kα inhibitors compared with single-hotspot mutations.