BACKGROUND: In Germany and Chile, substitutive private health insurance has been shaped by its co-existence with statutory social health insurance. Despite differences in the way choice is available to users in the health insurance regimes of Chile and Germany, the way in which each country has managed choice between private health insurance and statutory social health insurance provides a unique opportunity to comparatively assess the consequences of such an arrangement that has been previously underexamined. METHODS: We conducted a Most Similar Systems Design comparative policy analysis of the co-occurring private health insurance and statutory social health insurance systems in Germany and Chile. We describe and review the origins and development of the German and Chilean health care insurance systems with an emphasis on the substitutive co-existence between private health insurance and statutory social health insurance. We provide a critique of the market performance of the private health insurance regime in each country followed by a comparative assessment of the impact of private health insurance on financial protection, equity, and risk segmentation. RESULTS: Segmentation of insurance markets in both Germany and Chile has had significant consequences for equity, fairness, and financial protection. Due to market failures in health insurance and differences in the regulatory frameworks governing public and private insurers, the choice of public or private coverage has produced strong incentives for private insurers to select for risks, compromising equity in health care funding, heightening the financial risk borne by public insurers and lowering incentives for private insurers to operate efficiently. CONCLUSIONS: The degree of conflict arising from the substitutive parallel private health insurance system and the statutory social health insurance system varies between Germany and Chile, though policy goals remain similar. Recent reforms in both countries have attempted to improve the financial protection of the privately insured through regulation; nevertheless, concerns about risk segmentation remain largely unresolved.