OBJECTIVE: Vitamin D deficiency has been linked to a higher risk of cancer. There is insufficient data regarding the influence of treatment on vitamin D status. The aim of this study was to compare pre- and post-treatment levels of 25(OH)D in premenopausal and postmenopausal women with breast cancer with a different receptor status (ER-estrogen receptors, PR-progesterone receptors) and in healthy controls. METHODS: 49 patients with breast cancer (23 premenopausal), and 28 healthy controls matched for age, menopausal status and BMI. RESULTS: The pre-treatment levels of 25(OH)D in patients with breast cancer were significantly lower in comparison to the control group (19 ng/mL vs. 30 ng/mL, p<0.001), the lowest in premenopausal women (18.4 ng/mL). After the treatment period, a significant decrease in 25(OH)D level (mean 15.8 ng/mL) was observed. The pre-treatment level of 25(OH)D was significantly lower in patients with ER (16.1 vs. 23.9 ng/mL, p=0.02) and with PR (15 vs. 24.4 ng/mL, p=0.003). The mean pre- and post-treatment levels of 25(OH)D were lower in patients with Ki67 <21% (16.7 vs. 20.1 ng/mL, p=0.15; 12.5 vs. 18.1 ng/mL, p=0.02 respectively). CONCLUSIONS: 25(OH)D level is lower in patients with breast cancer in comparison to healthy women, regardless of their menopausal status. The lowest levels are found in patients with ER and PR positive tumours. While a significant decrease in 25(OH)D level during the course of therapy is observed, the supplementation of vitamin D should be considered.