Background: Our research study group recently evaluated a PLGS system embedded on the Tandem t:slim X2 with Basal-IQ insulin pump. The system was designed to work "in the background" without alarms when suspending and restarting insulin delivery. System usability and effectiveness in decreasing hypoglycemia are both critical to the success of a PLGS device. Methods: The PROLOG study was a randomized crossover trial conducted at 4 U.S. sites. Participants with type 1 diabetes (age ≥6 years, n=102) previously treated with MDI or CSII (with and without CGM) were randomized to the order of treatment: PLGS during one 3-week period and sensor-augmented pump (SAP) during the alternate 3-week period. We recently reported the primary outcome of the PROLOG study-a reduction of mean sensor time <70 mg/dL by 31% relative to SAP. In addition to glycemic outcomes, usability of the system was evaluated using a validated measure, the System Usability Scale (SUS). Results: The overall SUS score for at-home use of the Basal-IQ system was 88.8 (out of 100). A score above 68 is considered above average, and 88 is exceptional. Subgroup analyses revealed no differences related to age, baseline glycemic control or baseline diabetes therapy (MDI, CGM or pump use, Table 1). Conclusions: The t:slim X2 with Basal-IQ was safe, effective, and easy for participants to use, regardless of previous experience with diabetes technology. Disclosure J.E. Pinsker: Research Support; Self; Insulet Corporation, Dexcom, Inc., Tandem Diabetes Care, Inc.. Z. Li: None. B.A. Buckingham: Advisory Panel; Self; Novo Nordisk Inc., ConvaTec Inc.. Research Support; Self; Medtronic, Insulet Corporation, Dexcom, Inc., Tandem Diabetes Care, Inc.. Consultant; Self; Tandem Diabetes Care, Inc., Becton, Dickinson and Company. G.P. Forlenza: Advisory Panel; Self; Dexcom, Inc.. Research Support; Self; Medtronic, Tandem Diabetes Care, Inc., Insulet Corporation, Dexcom, Inc., Novo Nordisk Inc., Bigfoot Biomedical. E. Cengiz: Advisory Panel; Self; Novo Nordisk Inc.. Speaker's Bureau; Self; Novo Nordisk Inc.. Advisory Panel; Self; MannKind Corporation, ADOCIA, Arecor. M. Church: None. L. Ekhlaspour: None. R. Wadwa: Advisory Panel; Self; Eli Lilly and Company. Research Support; Self; MannKind Corporation, Dexcom, Inc., Xeris Pharmaceuticals, Inc., Bigfoot Biomedical. S.A. Weinzimer: Speaker's Bureau; Self; Medtronic MiniMed, Inc., Insulet Corporation. Consultant; Self; Sanofi. Stock/Shareholder; Self; InsuLine Medical Ltd.. C.C. Andre: None. T. Marcal: None. E. Jost: Other Relationship; Self; Medtronic MiniMed, Inc.. L.R. Carria: None. W.J. Woodall: None. B. Dokken: Employee; Self; Tandem Diabetes Care, Inc. V. Swanson: Employee; Self; Tandem Diabetes Care, Inc. J.W. Lum: Other Relationship; Self; Bigfoot Biomedical, Tandem Diabetes Care, Inc., Eli Lilly and Company, Ascensia Diabetes Care. C. Kollman: Research Support; Self; JDRF, Bigfoot Biomedical, Dexcom, Inc., Tandem Diabetes Care, Inc., Medtronic MiniMed, Inc., Helmsley Charitable Trust. R.W. Beck: Consultant; Self; Eli Lilly and Company. Research Support; Self; Abbott. Consultant; Self; Bigfoot Biomedical. Research Support; Self; Dexcom, Inc.. Consultant; Self; Insulet Corporation. Research Support; Self; Roche Diabetes Care Health and Digital Solutions. Consultant; Self; Merck & Co., Inc., Xeris Pharmaceuticals, Inc..