Lung involvement is a usual feature of connective tissue diseases. Different patterns of lung injury have been described in specific types of connective tissue diseases. Since the etiology and pathophysiology of connective tissue diseases are still unknown, we depend on constellation of defined clinical and laboratory findings to identify the specific connective tissue disease in a patient. Patients’ clinical manifestations may vary considerably within the defined connective tissue diseases, and in some instances, a patient may fulfill criteria for diagnosis of more than one connective tissue disease. It has been estimated that up to 25% of patients presenting with systemic connective tissue disease symptoms may not meet the criteria of any defined connective tissue diseases. The term undifferentiated connective tissue disease has been used to describe this group of patients. This term should be differentiated from two other terms: overlap syndrome and mixed connective tissue disease (Table 1
). Overlap syndrome is used when the patient meets the criteria for two or more connective tissue diseases. Mixed connective tissue disease is a specific syndrome with the overlap features of rheumatoid arthritis-like arthritis, myositis, scleroderma, and systemic lupus erythematosus features accompanied with high-titer antibodies to ribonucleoprotein. Bodolay et al observed 665 patients with undifferentiated connective tissue disease for 5 years. During the follow-up period, 34.5% of patients acquired a specific connective tissue disease (systemic lupus erythematosus, mixed connective tissue disease, scleroderma, polymyositis/dermatomyositis, and rheumatoid arthritis) or systemic vasculitis, 12.3% of patients had complete remission, and in 65.4% of patients undifferentiated connective tissue disease remained the main diagnosis. Although the presence of the sclerodactyly and elevated antinuclear antibody titer in this case suggests the presence of connective tissue disease, the clinical picture does not fulfill the criteria for any specific connective tissue disease.