Connective tissue disease (CTD) covers all heterogeneous and broad immunological diseases. These immunological diseases are characterized by inflammation, tissue damage, and abnormal repair. Disorders such as fibrotic tissue or loss of function are seen in the degeneration of the target organ. There is a complex relationship between genetic and environmental factors on the basis of these disorders. Vitamin D deficiency has been frequently observed in autoimmune diseases such as systemic lupus erythematosus, diabetes mellitus, and rheumatoid arthritis. Vitamin D deficiency is primarily seen with musculoskeletal complaints and is an early warning of osteomalacia. Muscle weakness is most common in the trunk, shoulders and hips. It is characterized by difficulty in climbing stairs, getting up from sitting or lying position, feeling of heaviness in the legs, duck-like gait, difficulty getting up from a chair, fatigue easily, inability to lift objects with arms and hands. Patients experience decreased exercise capacity as a result of pain and muscle weakness. In addition, vitamin D deficiency causes smooth muscle contraction and an increase in airway inflammation. As a result, it was observed that the disease course and symptoms were more severe in CVD patients with vitamin D deficiency. In a study conducted with rheumatoid arthritis patients, the quality of life, physical activity and depression levels of patients with vitamin D deficiency were compared with the control group. The evaluation parameters of rheumatoid arthritis patients were worse than the control group. Vitamin D levels were compared with sleep, anxiety and depression levels in patients with systemic lupus erythematosus. A positive relationship was found between vitamin D deficiency and sleep disturbance level. It has been observed that the life expectancy of patients with connective tissue disease is significantly reduced compared to healthy ones. Exercise capacity and oxygen consumption decreased in these patients.