In a retrospective study we analysed clinical findings and hematological data of 16 neonates born from mothers using methadone and other drugs during pregnancy. Most infants presented with symptoms of drug withdrawal and needed medication. Hemoglobin concentration and leucocyte counts were within normal limits, whereas all infants showed thrombocytosis after the first week of life with thrombocyte counts (median) of 566,750 (week 2), 628,500 (week 3), 629,500 (week 4), 617,000 (week 5) and 639,150/mm3 (week 6). No thrombocytosis was found in 18 normal control infants which showed significantly lower platelet counts during the first days (p < 0.05) and by the second week of life (p < 0.001). The cause of this phenomenon is not known. Sudden infant death syndrome occurred in 1 infant. There are speculations that the high incidence of SIDS in this group of patients could be explained by thromboembolic complications in some of the cases.