Recently, we have experienced significant number of patients diagnosed with non-specific interstitial pneumonia (NSIP) by open lung biopsy or video-assisted thoracoscopic surgery. The purpose of this study is to compare clinical and pathological features of idiopathic NSIP and NSIP associated with underlying diseases (mainly autoimmune disorders). Forty-six patients with histologically proven NSIP were retrospectively collected. Twenty-four patients had underlying diseases (12 polymyositis/dermatomyositis, 5 systemic sclerosis, 2 rheumatoid arthritis, 2 Sjogren's syndrome, 1 ulcerative colitis, 1 primary biliary cirrhosis, and 1 multiple myeloma). Twenty-two of the 46 patients had no underlying diseases. It was very difficult to distinguish idiopathic NSIP and NSIP associated with underlying diseases, clinically and radiologically. Pathologically, Lymphocytic pneumonitis was demonstrated in both groups, and it was impossible to distinguish idiopathic NSIP and NSIP associated with underlying diseases. Since generalized symptoms were not observed in patients with idiopathic NSIP, and clinical and pathological features were identical to NSIP with several autoimmune disorders, we postulate new clinical entities of "autoimmune interstitial pneumonia" in cases without underlying diseases.