Purpose: To evaluate effects of insulin on ocular parameters in patients with type 2 diabetes mellitus who start insulin therapy. Methods: In this prospective study, ocular biometric parameters were obtained using optical biometer (Lenstar LS900; Haag-Streit AG) and refraction test (ARK-510A Auto refracto-keratometer; Nidek Co. Ltd, Aichi, Japan) before and at 3 months after initiating insulin therapy. In addition, patients' fasting blood glucose (FBG), glycosylated hemoglobin (HbA1C), and blood lipid levels were measured at the same time points. Pretreatment and post-treatment results were compared. In addition, associations between ocular parameters with initial dose and type of insulin treatment regimen, HbA1C, and FBG levels were evaluated. Results: The patients' mean age was 51.2 +/- 12.9 (18-73) years. Post-treatment HbA1C and FBG levels (8.5% +/- 2.5% and 188.1 +/- 111.2 mg/dL, respectively) were significantly lower than pretreatment values (12% +/- 1.4% and 325.3 +/- 95.7 mg/dL, respectively; P < 0.001 for both). There was a significant positive correlation between the change in HbA1C and the change in lens thickness (P = 0.03), and a significant negative correlation between the change in FBG and the change in the spherical equivalent refraction (P = 0.045). Insulin dose and treatment regimen type were not significantly correlated with ocular parameters (P > 0.05). Conclusion: HbA1C-lowering glycemic effect of insulin was correlated with a small decrease in lens thickness. Long-term, randomized controlled trials including larger patient numbers are needed to shed light on the long-term effects of insulin use and glycemic control on ocular parameters.