Four cases of neonatal transient leukemoid reaction associated to Down's syndrome (TLRAD) studied in a Metropolitan Hospital at Santiago, Chile, since 1980 to 1985 are described. Diagnosis was done at the first week of live by blood cell counts in all the patients, and in every case these were characterized by high leukocyte counts (15,000 to 48,000 x mm3 blood), 13 to 25% circulating white blood cell blasts, 25 to 61% blast forms in bone marrow aspirates and normal erythrocyte and blood platelet series. In three cases the lower inferior hepatic border was palpated 3 to 5 cm under the rib's margin at the clavicular mid line. White blood cell blasts disappeared from circulation after 1 to 4 months in coincidence with reduction of liver's size to normal. Two of these patients died after remission of their leukemoid reaction as a result of serious systemic infections, without clinical nor laboratory evidence of leukemoid reaction neither leukemia, even though one of them was submitted to a complete postmortem examination. The other two cases had favourable clinical evolution and their hematologic controls have being normal along the following three years.