Objectives; The development of pulmonary hypertension(PHT) is considered a risk factor in the long-term course of patients with atrial septal defect(ASD). The aim of this study was to evaluate the electrocardiographic features of secundum ASD with PHT, compared to those without PHT in adults. Methods; Electrocardiography(ECG) was reviewed in 50 adults, mean age 34짹10 years(18- 62 years) with secundum ASD who underwent cardiac catheterization. Electrocardiographic features were analyzed with reference to the right ventricular hypertrophy(RVH) criterior in adult. PHT was defined as mean pulmonary artery pressure(PAP) over 30 mmHg. Results; The classical rsR��� QRS morphology in lead V1 were in 30 patients and the other 20 patients showed Rs morphology(RS). RS was more frequent in ASD with PHT(Group 1, n=20) than those without PHT(Group 2, n=30)(p=0.001). 9 patients with PHT showed small q wave before Rs, however, no one without PHT had qRs QRS morphology. In 20 patients with RS, height of R wave in V1(V1R) over 5.5 mV( p<0.01, 95% CI 0.808-1.05))and R/S ratio in V1(V1R/S) over 3.25( p<0.01, 95% CI 0.785-1.049) could discriminate the Group 1 from Group 2. In Group 1, V1R(r=0.50, p < 0.05) and V1R/S (r=0.56, p < 0.05) related with the degree of PAP. Among Group 1 patients, V1R over 10.5 mV( p<0.01, 95% CI 0.98-1.00))and R/S ratio in V1(V1R/S) over 5( p<0.01, 95% CI 0.91-1.045) could discriminate the patients with PAP over 50 mmHg. Conclusions; Most of ECGs in ASD patients with PHT showed the fetures of RVH. The indecies of RVH criterior such as height of R wave and R/S ratio in V1 were helpful to differentiate the patients with PHT from those without PHT and further to discriminate the patients with severe PHT.