Objective: Fatigue is a common symptom in diabetes mellitus. The aim of this study was to determine the factors leading to fatigue and to investigate the effect of insulin use on fatigue among the diabetic population. Material and Methods: One-hundred diabetic patients attending the diabetes clinic of Cerrahpasa Medical Faculty between October 2017-January 2018 and 42 healthy controls were evaluated in this cross-sectional study. Questionnaires including demographic and disease characteristics, Fatigue Impact Scale (FIS), Fatigue Severity Scale (FSS), Beck Depression Inventory (BDI), quality of life scale (SF-36), Epworth Sleepiness Scale (ESS), and Pittsburgh Sleep Quality Index (PSQI) were used. Results: Ages (47.6 +/- 14.8 and 45.7 +/- 14.1 years; p=0.47) and body mass indices (26.6 +/- 4.1 and 25.3 +/- 3.5 kg/m(2); p=0.08) of 100 patients with diabetes (Type 1 Diabetes/Type 2 Diabetes=29/71) and 42 healthy volunteers were similar. The diabetic group had worse FIS total (p=0.05), FIS psychological (p=0.04) scores and SF-36 scores compared to the healthy controls. When the patients with diabetes were divided into two groups according to insulin use and compared with healthy controls, the ESS and PSQI were similar but all FIS parameters (total p=0.005, cognitive p=0.007, physical p=0.01, psychological p=0.009) and BDI (p=0.05) were significantly worse in patients with insulin use than non-insulin and control groups. The relationship between fatigue and insulin use was independent of glycemic control and duration of diabetes but was affected by the BDI (p=0.001). Conclusion: Insulin use leads to fatigue in patients with diabetes, regardless of diabetes type, and this effect is influenced by depressive mood. Psychotherapeutic approaches prior to insulin treatment might yield fruitful results.