The government of Kenya is making plans to implement a social health insurance program by transforming the National Hospital Insurance Fund (NHIF) into a universal health coverage program. This paper examines the determinants associated with participation in the NHIF among residents of urban slums in Nairobi city.The study used data from the Nairobi Urban Health and Demographic Surveillance System in two slums in Nairobi city, where a total of about 60,000 individuals living in approximately 23,000 households are under surveillance. Descriptive statistics and multivariate logistic regression analysis were used to describe the characteristics of the sample and to identify factors associated with participation in the NHIF program.Only 10% of the respondents were participating in the NHIF program, while less than 1% (0.8%) had private insurance coverage. The majority of the respondents (89%) did not have any type of insurance coverage. Females were more likely to participate in the NHIF program (OR = 2.4; p < 0.001), while respondents who were formerly in a union (OR = 0.5; p < 0.05) and who were never in a union (OR = 0.6; p < 0.05) were less likely to have public insurance coverage. Respondents working in the formal employment sector (OR = 4.1; p < 0.001) were more likely to be enrolled in the NHIF program compared to those in the informal sector. Membership in microfinance institutions such as savings and credit cooperative organizations (SACCOs) and community-based savings and credit groups were important determinants of access to health insurance.The proportion of slum residents without any type of insurance is high, which underscores the need for a social health insurance program to ensure equitable access to health care among the poor and vulnerable segments of the population. As the Kenyan government moves toward transforming the NHIF into a universal health program, it is important to harness the unique opportunities offered by both the formal and informal microfinance institutions in improving health care capacity by considering them as viable financing options within a comprehensive national health financing policy framework.