OBJECTIVE: This study assessed the degree of implementation of the Breastfeeding-Friendly Primary Care Initiative in a Brazilian metropolis. METHODS: A tool with 55 items and a maximum score of 10 points, based on validated protocols, was developed for assessing the degree of implementation of the abovementioned initiative. This tool was used on a probabilistic sample of municipal primary care units in the city of Rio de Janeiro, Brazil (n=56). Managers (n=56), healthcare practitioners (n=541) and users (n=985) were interviewed. RESULTS: The mean score for the degree of implementation of the Breastfeeding-Friendly Primary Care Initiative in the study units was 5.45 (95%CI: 5.11 - 5.78), the maximum being 10.00. Existence of written guidelines and training obtained the worst scores. Breast massage and manual milk expression techniques; knowledge about contraception and the risks associated with the use of baby formulas, bottles, and pacifiers; and existence of support groups obtained intermediate scores. Provision of information and recommendations on breastfeeding rights and advantages; addressing users' worries, life experiences, and doubts; and encouraging cue feeding achieved the best scores. CONCLUSION: The proposed tool assessed the general degree of implementation of the Breastfeeding-Friendly Primary Care Initiative and of each group of actions associated with the "Ten Steps", providing information for restructuring the strategies used in Rio de Janeiro. The degree of implementation of breastfeeding promotion, protection, and support set by the initiative in this city is intermediate, with its items having been implemented to different degrees.