Human Respiratory Syncytial Virus (HRSV) is a leading cause of death among children and hospitalized infants with acute respiratory infection worldwide. HRSV is a non-segmented (-) ssRNA virus belonging to the Pneumoviridae family and to the genus Orthopneumovirus, being first described in 1956 at the Walter Reed Army Institute in the USA.The present study was conducted on nasopharyngeal aspirate samples collected from children up to 2 years of age hospitalized at the Santa Casa de Misericórdia Hospital in São Paulo and brings a retrospective evolutionary analysis of the HRSV fusion protein, emphasizing the region of the site of action of palivizumab which is currently the only prophylactic agent available for the infection caused by this virus.Mutations within site A have already been described as responsible for the emergence of resistant strains of palivizumab, so studies on monitoring the emergence of possible escape mutants for this mAB, as well as the pattern of distribution of this mutation among viral populations is of paramount importance. In this way analyzes of the frequency of F protein mutations were performed in the NCBI database, which showed the increasing distribution of the N276S mutation (which was a mutation first described in 2010) over the years in the GenBank sequences. The mutation was possibly fixed in subtype A, and although the mutation also occurs in the same position for subtype B (S276N), the latter occurs at an extremely low frequency. Genotyping revealed a cocirculation between the two subtypes of HRSV in the 3 seasonal seasons. In 2008, subtype A prevailed, whereas in 2009 (year in which the N276S mutation was inserted in the NA1 genotype), subtype B prevailed. In the year 2010 a greater balance was observed in the presence of subtypes and the mutation remains in the genotype NA1 in the last year studied. Among the subtypes, 3 genotypes were identified for HRSVA, GA2, GA5, NA1, and 3 genotypes for HRSVB, BA9, BA10, GB13. Within site II, the average similarity between the sequences was 96% at the nucleotide level and 100% at the amino acid level, demonstrating the high conservation status in the palivizumab region of activity, with 33 neutral polymorphic sites and 3 non-between the two subtypes, without the presence of indels.The selective pressure predictions performed in the Santa house sequences showed sites with possible purifying selection events in HRSVA and HRSVB. The analysis did not show selection event within the palivizumab region of action for the two subtypes. The dN / dS rate for subtype A was 0.0305 and for subtype B it was 0.0558, that is, both tools predicted negative selective pressure events in the two subtypes.The prediction of possible functional effects of mutations at site A and neighboring amino acids showed that there are regions in which amino acid changes would have a neutral effect, such as at sites 254-256. Feasible effects can be highlighted at sites 259, 267, 270 and 275. Already among the sites in which mutations would have the greatest effects on pr...