Osteomalacia is a metabolic bone disease, in which there is insufficient mineralization of the organic bone matrix in adults. The most common cause of osteomalacia is vitamin D deficiency. Reported symptoms are diffuse bone pain, fatigue and muscle weakness. Laboratory tests show lower levels of calcium and inorganic phosphate in the serum, an increase in alkaline phosphatase, urinary calcium excretion reduction, and lower level of vitamin D in serum. A case of osteomalacia in a 41-year-old woman in the course of severe vitamin D deficiency, with characteristic clinical and laboratory symptoms was presented. The severity of symptoms and ambiguous descriptions of imaging tests of skeletal system have led to the differential diagnosis taking into account neoplastic diseases. In the article it is shown, how important, in the diagnosis and cure of the patient, determining the level of vitamin D and its supplementation with calcium preparations turned out to be.