BACKGROUND: This study aimed to compare the hematological parameters of patients with very high and normal 25-hydroxyvitamin-D3 (25(OH)D3) levels. METHODS: This study was designed as a retrospective cross-sectional study. The patients were divided into three groups according to their 25(OH)D3 levels: groups 1, 2 and 3 consists of patients with normal 25(OH)D3 levels (30-88 ng/mL), hypervitaminosis D (89-149 ng/mL) and vitamin D intoxication (>150 ng/mL), respectively. According to vitamin D levels, statistical analysis was performed by comparing the biochemical and hematological data between the groups. RESULTS: This study evaluated 120 patients (40 patients) in three equal groups. A statistically significant difference was found between the three groups in hemoglobin (P=0.03), hematocrit (P=0.01), red blood cell levels (P=0.03), leukocyte count (P<0.001), neutrophil count (P<0.001), lymphocyte count (P=0.006), mean platelet volume (P=0.04), and neutrophil/lymphocyte ratio (P=0.03). In post-hoc analysis, hemoglobin, hematocrit and RBC were significantly higher in group 1 than in group 3 (post-hoc Tukey, P<0.05). A statistically significant negative correlation was noted between 25(OH)D3 level and hemoglobin (r=-0.236), hematocrit (r=-0.230), and red blood cell (r=-0.265) levels. CONCLUSIONS: Vitamin D intoxication has been observed to affect hemoglobin, hematocrit, and RBC levels negatively. However, more studies are needed to clarify the effects and mechanisms of high vitamin D levels on the hematopoietic system.