BACKGROUND: Maternal and Child Health is a global priority. Access and utilization of facility-based health services remain a challenge in low and middle-income countries. Evidence on barriers to providing and accessing services omits information on the role of security within facilities. This paper explores the role of security in the provision and use of maternal health services in primary healthcare facilities in Nigeria. METHODS: Study was carried out in Anambra state, Nigeria. Qualitative data were initially collected from 35 in-depth interviews and 24 focus groups with purposively identified key informants. Information gathered was used to build a programme theory that was tested with another round of interviews (17) and focus group (4) discussions. Data analysis and reporting were based on the Context-Mechanism-Outcome heuristic of Realist Evaluation methodology. RESULTS: The presence of a male security guard in the facility was the most important security factor that facilitated provision and uptake of services. Others include perimeter fencing, lighting and staff accommodation. Lack of these components constrained provision and use of services, by impacting on behaviour of staff and patients. Security concerns of facility staff who did not feel safe to let in people into unguarded facilities, mirrored those of pregnant women who did not utilize health facilities because of fear of not being let in and attended to by facility staff. CONCLUSION: Health facility security should be key consideration in programme planning, to avert staff and women's fear of crime which currently constrains provision and use of maternal healthcare at health facilities.