This randomized evaluation assessed the impact of extending health insurance to informal sector workers in Nicaragua through microfinance institutions (MFIs). The study randomly assigned subsidies for insurance premiums and locations for enrollment (at MFIs or the Social Security Institute). Insurance increased use of covered health facilities but not overall utilization. It reduced out-of-pocket costs but savings were lower than premiums. Retention rates were very low after subsidies expired. While insurance lowered financial barriers, uptake and continued participation remained challenging due to costs relative to incomes and perceptions of the value of insurance among informal workers.