• Age ≥ 14 years old at time of signing informed consent

• Able to provide informed consent (and assent for participants \<18 years old)

• Documentation of a CF diagnosis as evidenced by one or more clinical features consistent with the CF phenotype and one or more of the following criteria:

‣ Sweat chloride equal to or greater than 60 mmol/liter by quantitative pilocarpine iontophoresis test (QPIT) (when not taking a cystic fibrosis transmembrane conductance regulator (CFTR) modulator)

⁃ Two well-characterized mutations in the CFTR gene

• Clinical diagnosis of CFRD, defined as a person with CF and diabetes mellitus, treated with insulin for ≥3 months prior to screening

• Using the same insulin regimen for ≥1 month prior to screening and collection of baseline CGM data, with no plans to change regimen during the study: either multiple daily injections of insulin (MDI), basal-only without bolus insulin, an insulin pump without automation, or an automated insulin delivery (AID) system other than the BP (which is an exclusion)

• Total daily insulin dose must be ≥0.1 units/kg

• Able to speak and read English sufficient to understand the pump user interface and provide written materials for safe operation of the BP

• • For pediatric participants, this applies to both the participant and caregiver

• For participants \<18 years old, living with one or more parent/legal guardian knowledgeable about emergency procedures for severe hypoglycemia. A designated care partner must be willing to be linked to the participant's Dexcom Follow application with location sharing on.

• For participants \>18 years old who live alone, participant has a relative or acquaintance who lives within 30 minutes of participant and is willing to be contacted to check on participant if study staff feel that participant may be experiencing a medical emergency and cannot be reached. A designated care partner must be willing to be linked to the participant's Dexcom Follow application with location sharing on.

⁃ No use of a non-insulin glucose-lowering medication, except metformin, that is not approved for use in T1D within 3 months prior to signing informed consent and willing to not use any such medications during the course of the trial. Note: such drugs cannot be used even if prescribed for weight loss rather than glucose-lowering.

⁃ If not currently using a rapid-acting insulin that is approved for use in the iLet pump, willing and able to switch to an approved insulin when using the BP.

⁃ Participant has commercial glucagon available for treatment of severe hypoglycemia or will obtain it prior to randomization

⁃ Willing to authorize the study team to contact the participant's primary physician to inform them about their participation in this study.

⁃ Enrolled in the Cystic Fibrosis Foundation Patient Registry (participants may enroll in the Registry at the time of enrollment if not already enrolled).

⁃ No plans for trips of more than 14 consecutive days outside the United States during the period of study participation

⁃ Investigator believes that the participant can safely use the iLet and will follow the protocol • The investigator will take into account the participant's HbA1c level (there is no upper limit for eligibility), compliance with current diabetes management, prior acute diabetic complications, cognitive ability, and general medical condition. For this reason, there is no upper limit on HbA1c specified for eligibility.