Purpose: To evaluate the efficacy of intravitreal ranibizumab in the treatment of diabetic macular edema (DME) and parameters believed to affect prognosis. Materials and Methods: Patients who were given ranibizumab injections due to DME in the Department of Ophthalmology, at Cukurova University Medical Faculty Hospital, were included in the study. Treatment-naive patients comprised Group 1; those who had prior treatment other than ranibizumab comprised Group 2. The two groups were compared in terms of best corrected visual acuity (BCVA) and central macular thickness (CMT) in post-treatment examinations. Results: BCVA did not differ significantly between group 1 and 2 before (p=0.746) or after (p=0.468) treatment. Also, there was no significant difference between the two groups in terms of the change in BCVA after treatment (p=0.068). There was no significant difference between the two groups in terms of CMT before the treatment (p=0.167), whereas post-treatment CMT was significantly greater in Group 2 than in Group 1 (p=0.000). Group 1 showed a significantly larger decrease in CMT than Group 2 (p=0.041). Glycemic control method was significantly associated with change in BCVA and CMT in all eyes and in Group 1. Post-treatment decrease in CMT was significantly greater in patients who used oral antidiabetics compared to those who used insulin (p=0.010). Conclusion: We observed a relationship between glycemic control method and change in CMT in treatment-naive patients. The reduction in CMT was significantly more in patients using OADs compared to those using insulin. Awareness of the ocular and systemic factors that may affect prognosis is important when developing treatment regimens for DME.