NSIP associated with primary lung cancer has been rarely reported. In the present report, three cases of histologically proven non-specific interstitial pneumonia (NSIP) associated with primary lung cancer are described. Importantly, in our 3 cases, interstitial pneumonia which is histologically proven to be NSIP was observed diffusely in both lungs. NSIP in these 3 cases responded to steroid therapy. However, 2 patients died from primary lung cancer and 1 patient died from progression of the interstitial pneumonia. Although the association between lung cancer and NSIP has been rarely documented, this combination was considered to be one of the paraneoplastic phenomena. The possible association between primary lung cancer and NSIP is discussed.