This paper contributes to analysing and understanding the demand for (social) health insurance of informal sector workers in Kenya by assessing their perceptions and knowledge of and concerns regarding health insurance and the Kenyan National Hospital Insurance Fund (NHIF). It serves to explore how informal sector workers could be integrated into the NHIF. To collect data, focus group discussions were held with organized groups of informal sector workers of different types across the country, backed up by a self-administered questionnaire completed by heads of NHIF area branch offices. It was found that the most critical barrier to NHIF enrollment is the lack of knowledge of informal sector workers about the NHIF, its enrollment option and procedures for informal sector workers. Inability to pay is a critical factor for some, but people were, in principle, interested in health insurance, and thus willing to pay for it. In sum, the mix of demand-side determinants for enrolling in the NHIF is not as complex as expected. This is good news, as these demand-side determinants can be addressed with a well-designed strategy, focusing on awareness raising and information, improvement of insurance design features and setting differentiated and affordable contribution rates.