A case of juvenile myelomonocytic leukemia (JMML, previously referred to as JCML) in a neonate with Noonan syndrome (NS) is described. The boy presented with bilateral congenital hydrothoraces, nonimmune hydrops, dysmorphic facies, persistent thrombocytopenia, and leukocytosis. The diagnosis of JMML was made on bone marrow cell culture studies. Review of the literature reveals an unusual preponderance of hematologic malignancies, in particular JMML, among patients with NS. Of 40 NS patients admitted to the authors' institution during a 10-year period, there were 4 (10%) with evidence of a monocytic proliferation, which resolved spontaneously. The authors postulate that patients with NS may have an increased incidence of myeloproliferative disorders, which in most cases appears to be benign but may be preleukemic or even lethal.