- Background and objectives Alpha-1 antitrypsin (AAT) has been shown to reduce pro-inflammatory markers and protect pancreatic islets from autoimmune responses in recent studies. Our aim was to evaluate its safety and tolerability in three different doses, in a pediatric population with recent onset type 1 diabetes mellitus (T1DM). Methods A 37-wk prospective, open-label, phase I/II interventional trial, comprised of 24 recently diagnosed subjects (12 males; age 12.9 ± 2.4 yr), who received 18 infusions of 40, 60, or 80 mg/kg/dose high-purity, liquid, ready to use AAT over 28 wk (Glassia®; Kamada Ltd., Ness Ziona, Israel). Primary outcomes: safety and tolerability; secondary outcomes: glycemic control, C-peptide reserve, and autoantibody levels. Possible responders were defined as individuals with peak C-peptide that declined less than 7.5% below baseline. Results No serious adverse events, diabetic ketoacidosis (DKA), or severe hypoglycemic episodes were reported. Adverse events were dose-independent and transient. Glycemic control parameters improved during the study in all groups, independent of dosage. Hemoglobin A1c (HbA1c) decreased from 8.43 to 7.09% (mean, p < 0.001). At the end of the study, 18 subjects (75%) had a peak C-peptide ≥0.2 pmol/mL. Eight subjects (33.3%) were considered possible responders and were characterized by shorter duration of T1DM at screening (54.5 ± 34.3 vs. 95.9 ± 45.7 d, p = 0.036) and greater decrease in their HbA1c during the study period (−2.94 ± 1.55 vs.−0.95 ± 1.83%, p = 0.016). Conclusions AAT treatment was safe and well tolerated in pediatric subjects with recently diagnosed autoimmune diabetes. Placebo-controlled studies with larger cohorts and dose range are warranted in order to assess efficacy in maintaining pancreatic beta cell reserve and glycemic control.