This study examined hydroxyurea usage in young children with sickle cell anemia within New York State (NYS). The cohort was 273 children with sickle cell anemia born in NYS in 2006-2009 and enrolled essentially continuously in Medicaid for the first 4 years of life. Medicaid data were used to examine hydroxyurea usage in this group by age at first prescription fill, persistence, region, treatment institution, and year. Log-binomial regression models were used to estimate the likelihood of receiving hydroxyurea treatment. Data from birth through 2014 for all members of the study group were assembled and analyzed in 2015. About 25% of the cohort had at least one filled hydroxyurea prescription by their fifth birthday, and nearly 40% by the end of the study period. The mean proportion of days covered for the first year of therapy was 56.3%. Adherence was also assessed by calculating medication possession ratios for individual treatment periods. Slightly more than one third of treated children showed 80% coverage by these measures. There was a consistent, but not statistically significant, trend toward younger age at first fill. Significant regional and treatment center differences in initiation of hydroxyurea use, but not in persistence after initiation, were noted among NYS centers. Subsequent to clinical studies demonstrating safety, current NYS-wide use of hydroxyurea in young children with sickle cell anemia appears to be widespread and increasing. However, practice differences between treatment centers and inadequate adherence may limit the full disease-modifying effects of hydroxyurea.