A research report submitted to the Faculty of Health Sciences, University of the Witwatersrand Johannesburg, in fulfillment of the requirements for the degree of Master of Medicine in the branch of Paediatrics Johannesburg, 2017 / Introduction: Diarrhoeal illness is a major contributor to morbidity and mortality in children under five years. Hypernatraemia is a serious electrolyte disturbance associated with diarrhoea. There is a paucity of data of the incidence and possible risk factors of hypernatraemia in acute gastroenteritis amongst the paediatric population in South Africa. Objective: To document the incidence of hypernatraemia in children admitted with diarrhoea and any associations between hypernatraemia and potential risk factors. Method: This study used a prospective cross - sectional convenience sample of children between the ages of one and 24 months, who were admitted for diarrhoea complicated by dehydration. Caregivers were interviewed and demographic, clinical and laboratory variables were obtained. Results: A total of a 125 children were included into the study. Fifty one of the 125 children (41%) had hypernatraemia. Their serum sodium levels ranged between 154 and 171 mmol/l. Age below one year (p< 0.001) and severe dehydration (p = 0.003) were risk factors for hypernatraemia in the univariate analysis but only infancy remained significant after the multivariate analysis (Odds ratio 10.6, 95% CI: 3.5 – 32.6, p<0.001). Three patients demised and all were part of the hypernatraemic group (6% vs. 0%, p = 0.24). Hypernatraemia was significantly associated with neurological deficits (14% vs. 0%, p = 0.013). Conclusion: This study illustrated a high incidence of paediatric hypernatraemic dehydration, which has not been reported in other studies. An age of less than one year old was a statistically significant risk factor. / MT2017