This paper studies the role of private supplemental health insurance (SHI) in universal healthcare systems. Linking novel microdata on SHI contracts to rich administrative data from Sweden, we document a steep income gradient in take-up: higher-income individuals are substantially more likely to enroll in SHI despite a greater healthcare need among lower-income individuals. Exploiting variation in the timing of employer-sponsored SHI, we find large and persistent increases in healthcare utilization (23 percent). The effects are even larger for low-income individuals and extend beyond specialist consultations to high-value treatments, consistent with binding rationing in public care. Focusing on cancer as a high-stakes condition, we find that SHI increases screening and diagnoses and reduces mortality. Although SHI is privately contracted, its effects materialize largely within the public healthcare system: coverage increases publicly financed utilization and reduces waiting times, generating negative fiscal and congestion externalities.