Proprotein convertase 1/3 (PC1/3) deficiency is a very rare disease characterized by severe intractable diarrhea in the first years of life, followed by obesity and several hormonal deficiencies later. Diabetes mellitus requiring insulin treatment and diabetic ketoacidosis have not been reported in this disorder. We herein present a girl with PC1/3 deficiency who has been followed from birth to 17 years of age. She developed deficiencies of all pituitary hormones over time as well as diabetes mellitus while receiving growth hormone (GH) therapy. She was complicated with diabetic ketoacidosis during dietary management of diabetes mellitus, thus insulin treatment was initiated. Insulin requirement to regulate hyperglycemia was short-lived. Repeat oral glucose tolerance test five years later was normal. The findings of this patient show that diabetes mellitus can develop at any time during follow-up of cases with proportein convertase 1/3 deficiency especially under GH therapy.