Right ventricular (RV) and left ventricular (LV) diastolic stiffness may be independent contributors to disease progression in pulmonary arterial hypertension (PAH). The aims of this study are to assess reproducibility of peak emptying rate (PER) and early diastolic peak filling rate (PFR) for both the RV and the LV in PAH and study their relationship to stroke volume (SV). Triple weekly repetition of 20 (totalling 60) cardiovascular magnetic resonance (CMR) scans, were done on 10 patients with PAH and 10 healthy controls. RV and LV volumes were measured over the full cardiac cycle. PER and PFR were calculated as the first derivative of the time–volume relationship in both the RV and the LV and indexed to body surface area. Reproducibility and the relation to SV were studied in a mixed model. PFR was lower in PAH in both the RV (PAH = 170 mL/m²/s, controls = 236 mL/m²/s [p < 0.01]) and in the LV (PAH = 209 mL/m²/s, controls = 311 mL/m²/s [p < 0.01]). PERs were not significantly different between patients and controls. Reproducibility of PER and PFR was high. A trial targeting normalization of PFR requires a total sample size of < 20. PER and PFR in both ventricles were strongly associated with stroke volume (all four: p < 0.01). Biventricular diastolic dysfunctions are strongly associated with stroke volume, and CMR can quantify them with high reproducibility, enabling small sample sizes for trials of therapies targeting diastolic dysfunction to increase survival.