Background/Objectives: After pneumonectomy, the right ventricular stroke volume is pumped into pulmonary vessels whose volume has been reduced by approximately 50%. To sustain conditions for pulmonary flow, the flow reserve is increased in the remaining lung, which is conducive to the development of pulmonary hypertension symptoms. This study sought to examine pulmonary flow in one lung and the size of the right atrium (RA), right ventricle (RV) and pulmonary artery (PA) in patients who had undergone pneumonectomy and to establish the influence of time since pneumonectomy on these parameters, as well as their potential mutual dependencies. Methods: The retrospective analysis included 34 patients who had undergone pneumonectomy. Pulmonary flow was measured by means of perfusion scintigraphy. The diameters of the RA, RV and PA were evaluated based on computed tomography with contrast. Results: We observed complete or near-complete utilization of flow reserve in 38.2% (13/34) of patients, enlarged transversal and longitudinal dimensions of the RA in 17.6% (6/34) and 32.3% (11/34) of patients, respectively, and enlarged transversal and longitudinal dimension of the RV in 67.6% (23/34) and 44.1% (15/34) of patients, respectively. Dilatation of the PA was discovered in 23.5% (8/34) to 26.5% (9/34) of patients, as well as the presence of an extensive complex of radiographic features of pulmonary hypertension (PH) syndrome in 23.5% (8/34) of cases. Conclusions: Radiological features of PH were present in a significant number of patients. These features developed at varying rates but were present in all patients followed >10 years after the procedure.