BACKGROUND: We investigated associations between timing of sacubitril/valsartan initiation and postdischarge adherence among patients hospitalized for heart failure with reduced ejection fraction (HFrEF). Clinical trials support initiation of sacubitril/valsartan among patients hospitalized with HFrEF. The association between timing of initiation and postdischarge adherence is unknown. METHODS AND RESULTS: We analyzed patients hospitalized for HFrEF (EF of ≤40%) within the Get With The Guidelines Heart Failure registry linked with Medicare claims between October 2015 and September 2017 who were eligible for sacubitril/valsartan. Follow-up was through December 2018. Patients were grouped by timing of sacubitril/valsartan initiation. Sacubitril/valsartan adherence at 90 and 365 days after discharge was assessed by calculating proportion of days covered (PDC) using medication fills. Among 4666 patients, 108 (2.3%) were continued on sacubitril/valsartan (on sacubitril/valsartan at admission and discharge), 191 (4.1%) were initiated as inpatients, 130 (2.8%) were initiated at discharge, and 4237 (90.1%) were discharged without sacubitril/valsartan. Median (25th, 75th) proportion of days covered through 90 days among those continued, initiated as inpatients, and initiated at discharge was 0.9 (0.6-0.1), 0.3 (0.0-0.7), and 0.0 (0.0-0.7), respectively (P < .001). Patients discharged without sacubitril/valsartan had very low rates of any sacubitril/valsartan fills within 90 and 365 days of discharge (2.1% and 7.7% of surviving patients, respectively). CONCLUSIONS: In 2015-2017 US clinical practice, more than 90% of eligible patients hospitalized for HFrEF were discharged without sacubitril/valsartan. Patients initiated as inpatients had a higher postdischarge proportion of days covered than patients initiated at discharge. Patients discharged without sacubitril/valsartan were unlikely to receive it during follow-up. These findings highlight the importance of initiating sacubitril/valsartan during hospitalization to improve the quality of care.