Celhay P, et al. - Using multiple nationally representative surveys and administrative data sources spanning 2001–16, experts evaluated the impacts of Seguro Popular, implemented the Medical Insurance Century XXI (SMSXXI) on health outcomes and financial protection for Mexico's children. SMSXXI expanded access to covered interventions and supported better primary care. The programme also favored enhanced supply and quality of care by raising human and physical resources sensitive to unmet requirements. Improved resource availability and increased supply of healthcare, rather than elevated use, contributed to decreasing infant mortality and enhancing long-term health as proxied by self-reported morbidity and child height. Constant with the programme's focus on uninsured and low-income populations, the impact on mortality, long-term health status, and out-of-pocket health expenses were concentrated in vulnerable groups.