Vitamin D plays a role in maintaining calcium homeostasis, bone metabolism, the functioning of the immune, cardiovascular, and reproductive systems. A negative correlation between the serum concentration of vitamin D and the incidence of cancers, including breast cancer, has been shown. The study aimed to evaluate the serum vitamin D concentration in women after breast cancer treatment, the influence of the seasons, eating and social habits, vit. D supplementation, recommendations of the attending physicians on the current vit. D concentration, and the impact of determining its concentration on improving the level of this vitamin in the subsequent tests. The study was conducted among 94 women after radical breast cancer therapy. The patients' serum was tested twice for concentration of vit. D, and at the same time, a survey was conducted. The groups were tested every six months. Group A was first tested in winter, then in summer. Group B was included in the study in summer, and then it was tested again in winter. The control group, examined once, consisted of 94 women with a negative oncological history. In the population treated for cancer, 25(OH)D deficiency was significantly more frequent than in the general population. Supplementation of vit. D before the study was used by less than 50% of the patients, and the knowledge of recommendations regarding vit. D supplementati ; on was 22,6% and 12,5%, in groups A and B, respectively. Providing the results of vit. D concentration increased the percentage of patients supplementing it by about 30%, and the knowledge of the recommendations increased 3-4 times. Diet was not found to have a significant effect on the concentration of vitamin D. The optimal concentration of vitamin D in women after breast cancer therapy should be maintained through comprehensive care of medical personnel: providing information on the validity and benefits of maintaining an optimal 25(OH)D concentration, the methods of supplementation, and monitoring its concentration levels.