Vitamin D deficiency is associated with increased incidence of breast and colon cancer as well as with an unfavourable course of non-Hodgkin lymphoma. Vitamin D deficiency is common in cancer patients and is associated with poor cancer prognosis and disease progression. In breast cancer patients under polychemotherapy with anthracycline and taxane, a significant drop in 25(OH)D levels was observed. Osteo-malacia represents a new and previously unreported risk factor for the development of bisphosphonate-related osteonecrosis of the jaw. In vitamin D deficiency (until it is corrected) oral and parenteral bisphosphonates should not be used. Vitamin D status should be monitored in all cancer patients and treated by adequate vitamin D3 supplementation. This applies in particular to cancer patients with poor nutritional status, treatment with aromatase inhibitors, bisphosphonates, and CTX containing anthracycline, taxane and monoclonal antibodies as well as in cases of muscular or mucocutaneous disorders, fatigue and tumor cachexia.