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Introduction: In patients with isolated pulmonary stenosis (iPS), dilatation of the MPA has been reported, sometimes developing into large aneurysms (giantPA). The incidence, aetiology, natural history, and its treatment. Methods: We identified all patients with iPS that attended our tertiary cardiac centre (one of the largest in the UK) between 2003 and 2018 and reviewed clinical notes, echocardiography and MRI data. We evaluated the incidence of MPA dilatation in patients with iPS and its relation to age and pulmonary valve (PV) function. Results: 197 patients (107 children, 90 adults) with iPS were identified. 122 (62%) underwent balloon dilatation of PV for severe stenosis (median age 0.65 IQR 0.08-5.18 years), 30 (15%) underwent PV repair or replacement (median age of 2.06 IQR 0.9-5.2 years) and 45 (22.8%) did not require any procedure. In patients that did not require a procedure, mean peak velocity on echocardiography was 2.55±0.65 m/sec. In patients that required an intervention, the MPA was normal to mildly dilated in most patients (mean z-score +1.2±2.6). In patients who did not undergo any procedure, the size of the MPA was only weakly correlated with age (R2.10 p <.01) and severity of PV stenosis (p <.22). 12 patients (6% of total population) had giantPA (z-score > 4 in children, diameter > 50 mm in adults). Mean size of the largest diameter of the MPA was +5.8±1.9 z-score in children and 59±5 mm in adults. Of these patient 58% had undergone a procedure (mean age 11±14 yrs). Of note, while in the paediatric cohort the MPA size was easily obtained using echocardiography, in only 30% of adult patients the MPA was well visualized and there was a significant error (25±27%) in the provided measurements compared to 3D imaging obtained with MRI. In 7 of 8 adults with giantPA , diagnosis was made only with MRI. Conclusion: In patients with iPS, the incidence of giantPA is not negligible (at least 6%). MPA dilatation does not seem to correlate with PV function thus suggesting an underlying arteriopathy rather than a flow-related aetiology. In adults, MPA size is unreliably assessed using echocardiography. 3D imaging is therefore indicated in adults patients with iPS even in the presence of mild PV dysfunction. Total; word count : 1875 characters Figure 1: No impact of stenosis severity on MPA size in patient who did not undergo PV repair or replacement Figure 2: Aneurysmal dilatation of the MPA in a patient with isolated pulmonary stenosis (in this case mild stenosis). [ABSTRACT FROM AUTHOR]