Objective: to study the differences in the metabolism of vitamin D and calcium-phosphorus metabolism in patients with an active phase of acromegaly in comparison with healthy individuals. Materials and methods: The study included 8 patients with an active acromegaly, median age 36.5 ± 6.25 years, BMI 27.9 ± 1.95 kg/m2, IGF-1 907.3 ± 239 ng/ml, as well as 8 conditionally healthy individuals selected by age, sex and level of 25(OH)D determined by the immunochemiluminescent method (DEQAS certified). All participants were tested for calcium-phosphorus metabolism, PTH, and vitamin D metabolites by HPLC/MS-MS (25(OH)D3, 25(OH)D2, 3-epi-25(OH)D3 and 24,25(OH)2D3) before oral administration of 150 000 IU of an aqueous solution of cholecalciferol and 7 days after administration. Results: In the Acromegaly group, on the 7th day after taking the drug, there was a statistically significant increase in 25(OH)D3 (89.8 ± 10.5 vs. 54.1 ± 14.8 nmol/L), 3-epi-25(OH)D3 (9.0 ± 2.6 vs. 3.3± 1.1 nmol/L) and 24,25(OH)2D3 (8.3 ± 1.9 vs. 6.4 ± 2.1 nmol/L), and a decrease of 25(OH)D2 (0.8 ± 0.2 vs. 1.1 ± 0.3 nmol/L) and a ratio of 24,25(OH)2D3 to 25(OH)D3 (0.1 ± 0.02 vs. 0.13 ± 0.03). A statistically significant increase in albumin-adjusted calcium was also noted (2.39 ± 0.14 vs. 2.31 ± 0.13 mmol/L). The medians of the levels of PTH and phosphorus initially were 27.1 ± 13.5 pg/ml and 1.6 ± 0.3 mmol/l and did not change by day 7 after taking the drug; creatinine and magnesium levels also remained the same. The level of calcium-creatinine ratio in a single portion of urine (CCR) was initially within the reference interval for all patients, its median did not change by day 7, however, in two patients there was a clinically insignificant increase higher than the upper limit of the reference interval; the phosphorus-creatinine ratio in a single portion of urine increased significantly. In the control group, after taking cholecalciferol similar changes in the levels of the studied vitamin D metabolites were observed, the levels of PTH also remained the same, however, there were no changes in the median biochemical parameters of blood and urine by day 7 after drug intake. Among the studied vitamin D metabolites, there were initially no significant differences between the groups; on day 7 a difference was recorded for the level of 3-epi-25(OH)D3 (9.0 ± 2.6 in the Acromegaly group vs. 18.8 ± 8.9 nmol/L in the control group). Among the biochemical parameters in the Acromegaly group higher levels of ionized blood calcium (1.14 ± 0.05 vs 1.1 ± 0.03 mmol/L), blood phosphorus (1.61 ± 0.26 vs 1.15 ± 0.09 mmol/L) and CCR were observed. Conclusion: Loading dose of cholecalciferol in patients with acromegaly is associated with less production of 3-epi-25(OH)D3, and results in lower inactive fraction of vitamin D than in healthy controls. More studies are needed to evaluate the effect of 1.25(OH)2D3 level on calcium-phosphorus metabolism in acromegaly.