Costello syndrome (CS) is a rare multiple congenital anomaly/mental retardation syndrome characterized by coarse face, loose skin and cardiomyopathy. It is often associated with benign and malignant tumors. Several groups have now demonstrated that CS is caused by recurring mutations in the HRAS gene in different ethnic groups. Here, we describe three unrelated Dutch patients and show that they all have the same mutation, G12S, in HRAS. To our knowledge, our patients are the first Dutch to be analysed. The syndrome seems to be genetically homogeneous. We discuss the pertinent nosology of the syndrome.