BACKGROUND: We aimed to examine clinical features and outcomes of consecutive molecularly characterized patients with Noonan syndrome with multiple lentigines and hypertrophic cardiomyopathy. METHODS: A retrospective, longitudinal multicenter cohort of consecutive children and adults with a genetic diagnosis of Noonan syndrome with multiple lentigines and hypertrophic cardiomyopathy between 2002 and 2019 was assembled. We defined a priori 3 different patterns of left ventricular remodeling during follow-up: (1) an increase in ≥15% of the maximal left ventricular wall thickness (MLVWT), both in mm and z -score (progression); (2) a reduction ≥15% of the MLVWT, both in mm and z -score (absolute regression); (3) a reduction ≥15% of the MLVWT z -score with a stable MLVWT in mm (relative regression). The primary study end point was a composite of cardiovascular death, heart transplantation, and appropriate implantable cardioverter defibrillator-shock. RESULTS: The cohort comprised 42 patients with Noonan syndrome with multiple lentigines and hypertrophic cardiomyopathy, with a median age at diagnosis of 3.5 (interquartile range, 0.2–12.3) years. Freedom from primary end point was 92.7% (95% CI, 84.7%–100%) 1 year after presentation and 80.9% (95% CI, 70.1%–90.7%) at 5 years. Patients with MLVWT z -score >13.7 showed reduced survival compared with those with <13.7. During a median follow-up of 3.7 years (interquartile range, 2.6–7.9), absolute regression was the most common type of left ventricular remodeling (n=9, 31%), followed by progression (n=6, 21%), and relative regression (n=6, 21%). CONCLUSIONS: These findings provide insights into the natural history of left ventricular hypertrophy, and can help inform clinicians regarding risk stratification and clinical outcomes in patients with Noonan syndrome with multiple lentigines and hypertrophic cardiomyopathy.