Objective: We aimed to investigate the relationship between vitamin B12 deficiency and red cell distribution width- platelet ratio (RPR), and the variations in the parameters on vitamin B12 treatment. Methods: One hundred fifty-four patients with untreated vitamin B12 deficiency (56% men, mean age: 50 ± 12.7 years) (untreated group), 86 patients with vitamin B12 deficiency (62% men, mean age: 42 ± 20.7 years) on vitamin B12 treat- ment (treated group), and 92 age- and sex-matched control group (54% men, mean age: 45 ± 15.1 years) were included in the study. Hematological parameters were evaluated by the method of laser-based flow cytometric impedance, using an automated blood cell counter (ABX Pentra 120 Hematology Blood Analyzer). Results: RPR was significantly reduced in treated group compared with untreated group (4.88 ± 1.06; 6.13 ± 1.27; p<0.001, respectively). RPR, red cell distribution width (RDW) and mean platelet volume (MPV) were significantly differ- ent in untreated group compared to controls (p<0.05). There were no differences in platelet count in the three groups (p>0.05). Conclusion: We proposed that vitamin B12 deficiency has effects on RPR and supplementation with vitamin B12 cor- rects the RPR levels.