Objective To evaluate the relation between lung dosimetric parameters and the risk of symptomatic radiation pneumonitis (RP) in patients with non-small cell lung cancer (NSCLC) who had re-ceived postoperative radiotherapy. Methods From November 2002 to March 2006, 90 patients with NSCLC who had received postoperative 3-dimentinal conformal radiotherapy (3DCRT) were retrospectively analyzed, including 53 with stage ⅢA disease, 25 with stafe ⅢB disease and 12 with stage Ⅰ-Ⅱ disease but positive margins. Seventy (78%) patients underwent lobectomy, 20 ( 22% ) underwent pneumonectomy, and 38 ( 46% ) received adjuvant chemotherapy. The median radiation dose was 60 Gy given in 30 fractions of 2 Gy using 6 MV X-ray. The percentage of the whole lung volume ( Vx ) and the ipsilateral absolute lung volume ( Vipsi-dosewhich received more than a certain dose were calculated. The endpoint was grade 2 and above RP based on CTC AE 3.0. The relation between the dosimetric factors and RP was also analyzed with receiver operating characteristic (ROC) curves. Results Nine patients ( 10% ) developed symptomatic RP ( grade 2 in 7 and grade 3 in 2), and all of them were in the lobectomy group. No RP was observed in patients who received pneumonectomy. Both V30 and V35 were higher in patients with RP than those without ( 19% vs 14% ,U= -2.16,P=0.030, and 15% vs 11%,U= -2.65,P =0.007, respectively). The area under curve in receiver operating characteristic curves based on the relation between incidence of RP and the value of Vipsi-dose was 0. 757. Using Vipsi-30 of 340 cm3 as a cut-off to predict RP, the sensitivity and specificity were 88% and 70%, respectively. The incidence of RP was 3% in patients with Vipsi-30< 340 cm3 compared with 29% in those with Vipsi-30>340cm3 ( X2 = 9.75 , P = 0.003 ) . Conclusions More than340 cm3 of the ipsilateral lung receiving 30 Gy is significantly related to the risk of RP in patients undergoing lobectomy. It is safe for patients who undergo pneumonectomy to receive postoperative 3DCRT if lung V20 is less than 10%.