Noonan syndrome (NS) is an autosomal dominant disorder characterized by multiple dysmorphic features and a broad spectrum of congenital heart defects. Specific mutations of the PTPN11 gene are associated with 50% of the NS cases and 90% of the multiple lentigines/LEOPARD syndrome (ML/LS) cases. These two allelic conditions have several overlapping clinical features. This study describes the association between the Gln510Glu mutation of the PTPN11 gene and lethal progressive hypertrophic cardiomyopathy (HCM) in a newborn with the NS phenotype. The findings confirm the intriguing relationship between site-specific mutations of the PTPN11 gene and rapidly progressive HCM.