We aimed to compare the effectiveness of bilateral and unilateral block application in chronic migraine patients and whether there were differences in their effectiveness retrospectively. In chronic migraine patients undergoing Greater occipital nerve (GON) block, mean number of days with pain per month before and after block, mean duration of pain in attacks (in hours), and mean Visual Analog Scale (VAS) in attack and pain severity were recorded from files. The patients underwent one block a week for the first 1 month, thereafter one block a month according to GON block protocol used by our institute. Of 41 patients included in the study, 23 underwent unilateral block (group 1) and 18 underwent bilateral block (group 2). In both groups, number of days with migraine decreased significantly in 2 and 3 months as compared to pre-block treatment (P < 0.001). Mean duration of headache decreased in group 2 during treatment (P < 0.001). In group 1, mean duration of headache also decreased but did not differ significantly (P = 0.051). Mean severity of migraine decreased significantly differ in group 1 in 2, 3 months as compared to pre-block treatment (P < 0.001). No differences were observed in frequency, severity and duration of headache between groups during 3-month treatment period. GON block is effective in chronic migraine and bilateral application is no superior over unilateral application.