© 2021 College of Physicians and Surgeons Pakistan. All rights reserved.Objective: To determine the effect of vitamin D treatment on platelet counts according to age groups in patients with vitamin D deficiency. Study Design: An observational study. Place and Duration of Study: Outpatient Clinics, Department of Internal Medicine, Faculty of Medicine, Lokman Hekim University Hospital, Ankara, Turkey from January 1st to December 31st, 2020. Methodology: A total of 1,230 participants, who were treated for vitamin D deficiency after admission to hospital for various reasons, were enrolled. Patients under 18 years of age, having chronic diseases, using medicines affecting platelets and who had 25-hydroxyvitamin D levels above 74.88 nmol/L were excluded from study. Participants were divided into three groups according to ages: 18 to 30 (Group 1), 30 to 50 (Group 2), and greater than 50 years (Group 3). Complete blood count parameters and vitamin D level data before and after vitamin D replacement treatment were compared. Results: Platelet counts and mean platelet volume levels were found to be significantly lower in participants after vitamin D treatment. Changes in hemoglobin and neutrophil counts were insignificant. Although there was no difference between mean platelet counts and age groups before treatment, significant differences were noted after vitamin D treatment (p=0.004). Correlation between vitamin D levels and changes in platelet levels compared to treatment beginning was found to be significant (r=-0.159, p<0.001). Conclusion: Vitamin D treatment lowered platelet counts. This may be beneficial in medical conditions such as essential thrombocythemia in which platelet counts are higher than normal, and may help decrease platelet counts.