We report the case of a child with clinical and haematological features indicative of juvenile myelomonocytic leukaemia (JMML). The patient showed dysmorphic features: high forehead, bilateral epicanthal folds, long eyebrows, low nasal bridge and slightly low-set ears. A 38G>A (G13D) mutation in exon 1 of the NRAS gene was first demonstrated on peripheral blood cells, and then confirmed on granulocyte-macrophage colony-forming units. The same mutation was also found in buccal swab, hair bulbs, endothelial cells, skin fibroblasts. This case suggests for the first time that constitutional mutations of NRAS may be responsible for development of a myeloproliferative/myelodysplastic disorder in children.