male or female, age at least 18 years at the time of signing inform consent

type 1 diabetes mellitus (diagnosed clinically) for at least 12 months at the time of screening (visit 1)

currently treated with insulin aspart, insulin lispro or insulin gluisine for at least 3 months prior to screening (visit 1)

using an external csii system for the previous 6 months prior to screening (visit 1)

hba1c (glycosylated haemoglobin) below or equal to 9.0% as assessed by central laboratory

body mass index (bmi) 20.0-35.0 kg/m^2

history of diabetic ketoacidosis (dka) episodes requiring hospitalization within 6 months prior to screening (visit 1)

history of abscess at the infusion site within 6 months prior to screening (visit 1)

hypoglycaemic unawareness as judged by the investigator or history of severe hypoglycaemic episodes requiring hospitalization within the last 6 months prior to screening (visit 1)