Title: Cost-effectiveness analysis of the Baby-Friendly Hospital Initiative to promote breastfeeding and reduce infant mortality INTRODUCTION: Breastfeeding practice has the greatest impact on the prevention of infant morbimortality. The Baby-Friendly Hospital Initiative (BFHI) is an important strategy on the promotion of breastfeeding and its costs might justify to study it to evaluate effectiveness for the health system. OBJECTIVES: To evaluate the cost-effectiveness of BFHI in promoting exclusive breastfeeding and reducing infant mortality in the city of São Paulo. METHODOLOGY: This is a study of economic evaluation of public health policy with data from a prospective cohort study conducted in six public hospitals. The intervention group was composed of three Baby-Friendly Hospitals (BFH) and the comparison group by three Not Baby-Friendly Hospitals (NBFH). The effectiveness of BFHI was evaluated by analyzing intermediate outcomes: the increase first-hour breastfeeding, breastfeeding (BF), exclusive breastfeeding (EBF) and final outcomes: avoided cases of late neonatal deaths, at six months and hospitalizations. The outcomes were estimated by calculating the Preventive Fraction (PF) for neonatal mortality and hospital admissions, and the Risk Reduction Ratio for estimating infant mortality in children under six months. The average costs according to type of delivery and length of stay were calculated, by micro-accounting for the cost items: medicines, supplies, exams and human resources to obtain the cost-Effectiveness Ratio (CER) and incremental Cost-Effectiveness Ratio (ICER) for the intermediate and final outcomes. Tests for comparison of proportions and means were applied. The Poisson regression was adjusted according to maternal age and schooling (significance of 95% (α = 5%)) and univariate and deterministic sensitivity analysis to evaluate the robustness of the ICER. RESULTS: 969 mothers were included in the first interview, 902 in the second and 814 in the third. Among children born in BFH, the prevalence of breastfeeding was higher in all indicators compared to those born in NBFH: 30.7% higher for first-hour breastfeeding; 1.2% in EBF at 30 days; 7.2% in BF and 21.9% in EBF at 6 months of life. Only the first-hour breastfeeding increment showed a statistically significant difference between BFH and NBFH. After adjustment of variables firsthour breastfeeding was negatively associated with: born in NBFH (RP1,29 IC1,04-1,59), cesarean delivery (RP1,90 IC 1.53-2,36) and did not make skin-to-skin (RP4,13 CI 2,38-7,13). EBF at 30 days was negatively associated with the mother having had difficulty in breastfeeding (RP1,38 CI 1.03-1.85), not living with a partner (RP1,46 CI 1.08-1.96), not having received support (RP1,40 CI1,01-1,95) non-compliance with step 6 (RP1,86 CI 1.36-2.55), noncompliance with step 9 (RP1,95 IC 1.32-2 , 88) and children use pacifiers (RP1,84 IC1,39-2,42). EBF at 180 days were negatively associated: use of pacifiers (RP 1.19 CI 1.02-1.39) and maternal work (RP1.27 CI 1.08-1.49). The s...