To study safety, efficacy and weight loss with ADO09, a co-formulation of insulin A21G and pramlintide, in type 1 diabetes. A randomized, two-arm ambulatory 16-week study compared ADO09 with insulin lispro in 80 participants with type 1 diabetes. We compared changes of weight, glycated haemoglobin, glycaemic patterns during continuous glucose monitoring, and insulin doses at baseline and at the end of treatment. A significant and continuing weight loss, the primary endpoint, was observed with ADO09 compared with lispro as prandial insulin. In the whole group, the weight loss with ADO09 relative to lispro was 2.1 kg. Glycaemic control was relatively good (7.7% mean glycated haemoglobin) in both groups and did not change during treatment. Prandial insulin doses were reduced by 21% in the ADO09 group, whereas basal insulin dosage was not modified. Gastrointestinal symptoms were more frequent with ADO09, but no clear difference in hypoglycaemia was observed. These results extend previous observations on the efficacy and safety of this insulin/pramlintide co-formulation. They show a beneficial effect on weight, using less mealtime insulin and without increased hypoglycaemia.