An infant affected by Noonan syndrome and presenting with abnormal hemopoiesis is described. Cytogenetic analysis, molecular studies, and in vitro culture assays ruled out myelodysplasia or other hematologic malignancies but were consistent with transient abnormal myelopoiesis. At present, 3 years and 10 months after diagnosis, the patient shows a normal hematologic picture. Supportive treatment in such patients, unless refractory cytopenias or overt leukemia develops, is emphasized.