As a growing number of low- and middle-income countries commit to achieving universal health coverage, one key challenge is how to extend coverage to informal sector workers. Micro health insurance (MHI) provides a potential model to finance health services for this population. This study presents lessons from a pilot study of a mandatory MHI plan offered by a private insurance company and distributed through a microfinance bank to urban, informal sector workers in Lagos, Nigeria.Study methods included a survey of microfinance clients, key informant interviews, and a review of administrative records.Demographic, health care seeking, and willingness-to-pay data suggested that microfinance clients, particularly women, could benefit from a comprehensive MHI plan that improved access to health care and reduced out-of-pocket spending on health services. However, administrative data revealed declining enrollment, and key informant interviews further suggested low use of the health insurance plan. Key implementation challenges, including changes to mandatory enrollment requirements, insufficient client education and marketing, misaligned incentives, and weak back-office systems, undermined enrollment and use of the plan.Mandatory MHI plans, intended to mitigate adverse selection and facilitate private insurers' entry into new markets, present challenges for covering informal sector workers, including when distributed through agents such as a microfinance bank. Properly aligning the incentives of the insurer and the agent are critical to effectively distribute and service insurance. Further, an urban environment presents unique challenges for distributing MHI, addressing client perceptions of health insurance, and meeting their health care needs.