If lithium therapy is required during pregnancy or lactation, serum lithium monitoring may be indicated in the newborns. In the neonatal population, however, blood samples are often obtained with a sampling device containing lithium heparin. Given the infrequent nature of lithium measurement in the neonatal population, a risk of oversight on the use of these lithium-containing devices for lithium measurements exists. Two such neonatal cases are reported, which may have been mismanaged if the measured levels of lithium were not suspected to be spurious. Patient 1 was a 3-day-old infant with a congenital heart disease born at 31 weeks' gestation to a mother on lithium. After a surgical repair, because of a potential need to monitor lithium in the infant's serum during breastfeeding, a remaining sample of apparently serum from a previous blood testing on day 1 of life was measured for lithium as a baseline value. Despite the lack of toxic signs, the result showed a toxic lithium level of 4.19 mmol/L. Lithium levels in follow-up samples were 0.11 mmol/L (day 4) and undetectable (day 6). Patient 2 was a full-term infant exposed to maternal lithium throughout the fetal life and breastfeeding. Serum lithium at day three of life in a hospital was undetectable, but after discharge, the lithium concentrations increased to 1.1 mmol/L on day 18, with no sign of lithium toxicity or renal dysfunction. This raised a possibility of significant exposure through breastfeeding, but later they were found to be spurious due to the use of lithium-containing devices. Separate investigation of effects of sampling devices on lithium levels indicated that levels of >3 mmol/L were possible if a sampling volume of blood was substantially small compared with the capacity of a tube containing lithium heparin. Serum lithium at day 3 of life in a hospital was undetectable, but after discharge, the lithium concentrations increased.