We report four cases of Noonan syndrome associated with chronic myelomonocytic leukemia in childhood. These children shared some hematologic features: thrombocytopenia, splenomegaly in the first months of life, occurrence of chronic myelomonocytic leukemia without abnormalities of the initial bone marrow karyotype, and, in three cases, improvement of the hematologic disease. A common pathophysiologic process in such patients is suggested.