Insulin resistance is highly prevalent in pulmonary arterial hypertension (PAH) patients. However, the long-term impact of diabetes mellitus (DM) on survival in PAH is unclear. Insulin resistance and DM are associated with left ventricular steatosis and dysfunction, but whether the right ventricle (RV) might be affected by DM in PAH is unknown. We hypothesized that PAH patients with DM would have worse survival than PAH patients without DM and that this would be due to impaired RV compensation. From a large registry of PAH patients at our institution, we analyzed the effect of DM on survival in patients with idiopathic or heritable PAH. Clinical and hemodynamic differences were compared between PAH patients with DM and those without DM. Twenty-nine patients with DM and 84 without DM were included. Gender, body mass index, PAH type and duration, and 6-minute walk distance were similar between groups. PAH patients with DM had significantly lower survival at 10 years than PAH patients without DM. Right atrial pressure, pulmonary arterial pressure, and cardiac output did not differ significantly between the two groups. However, right ventricular stroke work index (RVSWI) was lower in the PAH DM group than in the no-DM patients. Among PAH patients with DM, patients who died had a lower RVSWI than survivors. In conclusion, survival in PAH patients with DM was reduced compared to that of patients without DM; impaired RV compensation may underlie this finding. Further study is needed to understand this effect.