BACKGROUND:Hypothermia on admission is associated with increased mortality in preterm infants. Drugs administered to pregnant women is implicated in its occurrence. Since magnesium sulfate has a myorelaxant effect, we aimed evaluating the association of hypermagnesemia at birth and admission hypothermia (axillary temperature <36.5°C) in preterm infants. METHODS:We performed a secondary analysis of a prospective cohort study database including inborn infants <34 weeks, without congenital malformations. Hypermagnesemia was considered if the umbilical magnesium level > 2.5 mEq/L. Maternal and neonatal variables were used to adjust the model, submitted to the multivariate hierarchical modelling process. RESULTS:We evaluated 249 newborns with median birth weight and gestational age of 1375 (IQR 1020–1375) g and 31 (IQR 28–32) weeks, respectively. Hypermagnesemia occurred in 28.5% and admission hypothermia occurred in 28.9%. In the univariate analysis, the following variables were identified as being associated with admission hypothermia: hypermagnesemia (OR 3.71; CI 2.06–6.68), resuscitation (OR 2.39; CI 1.37–4.19), small to gestational age (OR 1.91; CI1.03–3.53), general anesthesia (OR 3.34; CI 1.37–8.13), birth weight (OR 0.998; CI 0.998–0.999) and gestational age (OR 0.806; CI 0.725–0.895). In the hierarchical regression model, hypermagnesemia remained independent associated with admission hypothermia (OR 3.20; CI 1.66–6.15), as well as birth weight (OR 0.999; CI 0.998–0.999) and tracheal intubation (3.83; CI 1.88–7.80). CONCLUSION:Hypermagnesemia was associated with an increased risk of admission hypothermia, as did tracheal intubation and lower birth weight.