ABSTRACT Acute respiratory tract infections, such as bronchiolitis and pneumonia, are the leading cause of emergency department (ED) visits and hospitalizations in US children, accounting for one in 5 acute care encounters. Out- of-home daycare is a well-established risk factor for these infections in young infants, and a key factor driving early daycare exposure is lack of paid family leave, with 50% of US women having no paid family leave benefit. In 2018, New York State introduced the nation’s most comprehensive parental leave policy, providing up to 8 weeks’ paid leave, enabling new mothers to delay their return to work until their newborn infant is at least 8 weeks old and thus delaying the start of out-of-home childcare. We propose to evaluate whether this policy helped to reduce rates of acute care encounters (ED visits or hospitalizations) for respiratory tract infection in young infants (up to 8 weeks of age). We will conduct a controlled interrupted time series analysis to measure the effect of New York’s paid family leave on acute care encounters for respiratory tract infection in young infants, using other Northeastern states as controls. We focus on New York State’s paid leave because at the time of its introduction on Jan 1, 2018, New York had the most generous policy in the country, with the broadest eligibility criteria and the highest uptake rate, maximizing the chance of detecting policy benefits. We will obtain all-payer hospital discharge and emergency care visit records for infants ≤8 weeks old in New York State, Oct 2015- Feb 2020, as well as comparable records from 4 control states that do not have paid family leave: New Hampshire, Maine, Vermont, and Maryland. We will identify acute care encounters for respiratory tract infections using international classification of diseases (ICD)-10 diagnosis coding. We will implement our time series analysis using negative binomial regression, accounting for the strong seasonality of respiratory infections using harmonic functions or indicator variables. We will use a similar time series design to examine changes in disparities by race-ethnicity, insurance type (as a proxy for family income), and quintiles of the Childhood Opportunity Index 2.0 (a global measure of relative neighborhood disadvantage). Our study’s findings can provide high-quality evidence on the effects of US paid leave on child respiratory tract infections, and if the introduction of paid family leave reduced or exacerbated existing differences in acute care encounters for these infections. These findings may be particularly useful to policymakers considering implementing or expanding paid family leave policies in their state, as improving children’s health is a shared priority of policymakers with different political ideology.