Accidental hypothermia in mature neonates requiring hospitalization is a clinical complication that has not been studied in detail. In the present study, the effect of accidental hypothermia on common morbidity in neonates in the postnatal period was investigated. We did temperature measurements in 616 mature neonates after birth, after transfer to the maternity ward and at check-up at age of 48-72 hours of life. Additionally, nurses checked temperature until discharge at every diaper change in 302 of all included neonates who were small for gestational age (SGA) or whose mothers had green fluid or premature rupture of membranes (PROM). We investigated if there was a correlation between infection, hyperbilirubinemia, hypoglycemia, SGA, gestational diabetes, PROM, green amniotic fluid, positive GBS status of the mother, and accidental hypothermia in the first 48-72 hours of life. Mature neonates showed transient accidental hypothermia in the first 24 hours of life. No significant correlation between infection (p=0.571), hyperbilirubinemia (p=0.837), hypoglycemia (p=0.072), and accidental hypothermia could be seen. There was a significant correlation between SGA (p=0.020), PROM (p=0.008), and accidental hypothermia, while hyperthermia was associated with infection (p=0.009) and green amniotic fluid (p=0.004). SGA and PROM represent perinatal risk factors for postnatal transient accidental hypothermia in mature neonates. Increased morbidity or mortality associated with postnatal accidental hypothermia has not been demonstrated.