This study aimed to investigate risk factors for poor outcomes in infants with isolated atrial septal defect (ASD) and pulmonary hypertension who had unexpectedly fatal course. We retrospectively reviewed 22 infants with isolated ASD and pulmonary hypertension, and analyzed the relationship between clinical outcomes and pulmonary hemodynamic parameters including pulmonary arterial resistance (Rp) and compliance (Cp) based on cardiac catheterization among them. Age and weight at cardiac catheterization were 5 (1–11) months and 4.9 (3.1–9.2) kg, respectively. There were 17 individuals with Down syndrome. Pulmonary hemodynamic parameters were shown as follows: the ratio of pulmonary to systemic blood flow: 2.0 (0.6–3.8), mean pulmonary arterial pressure: 41 (20–60) mmHg, the ratio of pulmonary to systemic mean pressure (Pp/Ps): 0.67 (0.46–1.13), Rp: 4.11 (0.68–15.80) Wood units m2, and Cp: 1.80 (0.63–6.16) mL/mmHg m2. There were 4 deaths during the follow-up period of 40 (7–241) months. Univariable logistic regression analysis revealed that Pp/Ps (odds ratio [OR]: 18,500, 95% confidential interval [CI] 1.48–230,659,690, P = 0.041) and Cp (OR: 0.03, 95% CI 0.001–0.73, P = 0.031) were significantly related to death. A Pp/Ps cutoff value more than 0.94 and a Cp cutoff value less than 0.97 mL/mmHg m2 yielded as the predictors of death with sensitivity of 98% and 94%, specificity of 75% and 75%, respectively. Decreased Cp is a previously unrecognized predictor for poor outcome in infants with isolated ASD and pulmonary hypertension.