Increasing awareness of possible pulmonary involvement in patients with systemic lupus erythematosus (SLE) will enable earlier diagnosis of the disease and minimize damage accumulation. According to our research, no study has been found that examines respiratory functions in detail in SLE patients and investigates their relationship with functional capacity, physical activity, and fatigue levels. Our primary aim is to compare respiratory functions, functional capacity, physical activity, and fatigue levels between patients with SLE and healthy controls. Our secondary aim is to investigate the relationship between respiratory functions with functional capacity, physical activity levels, and fatigue in SLE patients.