Atelectasis was reported as a favorable prognostic sign of pulmonary carcinoma; however, the underlying mechanism in those patients is not known. In this study, we aimed to investigate prospectively the potential impact of atelectasis and/or obstructive pneumonitis (AO) on survival and the relation between atelectasis and some laboratory blood parameters. The study was conducted on 87 advanced stage non-small cell lung cancer (NSCLC) patients. Clinical and laboratory parameters of patients at first presentation were recorded, and patients were divided into two groups according to the presence of AO in thorax computed tomography (CT). Survival was calculated using Kaplan-Meier and univariate Cox's regression analyses. Laboratory parameters that might be related with prolonged survival in atelectasis were compared using chi-square, Student's t, and Mann-Whitney U tests. Of the patients, 54% had stage IV disease, and AO was detected in 48.3% of all cases. Overall median survival was 13.2 months for all cases, 10.9 months for patients without AO, and 13.9 months for patients with AO (P = 0.067). Survival was significantly longer in stage III patients with AO (14.5 months versus 9.2 months, P = 0.032), but not in stage IV patients. Patients with AO in stage III had significantly lower platelet counts (P = 0.032) and blood sedimentation rates than did those with no AO (P = 0.045). We concluded that atelectasis and/or obstructive pneumonitis was associated with prolonged survival in locally advanced NSCLC. There was also a clear association between atelectasis and/or obstructive pneumonitis and platelets and blood sedimentation rate.