Vitamin D deficiency and insufficiency are highly prevalent in CKD affecting over 80% of hemodialysis (HD) patients and require therapeutic intervention. Nephrological societies suggest the administration of cholecalciferol according to the guidelines for the general population. The aim of the observational study was to evaluate the efficacy and safety of the therapy with a high dose of cholecalciferol in HD patients with 25(OH)D deficiency and insufficiency to reach the target serum 25(OH)D level > 30 ng/mL. 22 patients (16 M), with an average age of 72.5 ± 13.03 years and 25(OH)D concentration of 13.05 (9.00–17.90) ng/mL were administered cholecalciferol at a therapeutic dose of 70,000 IU/week (20,000 IU + 20,000 IU + 30,000 IU, immediately after each dialysis session). All patients achieved the target value >30 ng/mL, with a mean time of 2.86 ± 1.87 weeks. In the first week, the target level of 25(OH)D (100%) was reached by 2 patients (9.09%), in the second week – by 15 patients (68.18%), in the fourth week – by 18 patients (81.18%) and in the ninth week by all 22 patients (100%). A significant increase in 1,25(OH)2D levels was observed during the study, however, only 2 patients (9.09%) achieved a concentration of 1,25(OH)2D above 25 ng/mL – the lower limit of the reference range. The intact PTH concentrations remained unchanged during the observation period. No episodes of hypercalcemia, and one new episode of hyperphosphatemia were observed. In conclusion, our study showed that the administration of a high therapeutic dose of cholecalciferol allowed for a quick, effective, and safe leveling of 25(OH)D concentration in HD patients.