Aims To describe incidence, presentation, treatment and short term outcomes of severe neonatal hypernatraemia (SNH, sodium ≥160 mmol/l).

Methods Prospective, population based surveillance study over 13 months using the British Paediatric Surveillance Unit. Cases were >33 weeks gestation at birth, fed breast or formula milk and <28 days of age at presentation.

Results Of 62 cases of SNH reported (7, 95% CI 5.4 to 9.0 per 1 00 000 live births), 61 mothers had intended to achieve exclusive breast feeding. Infants presented at median day 6 (range 2–17) with median weight loss of 19.5% (range 8.9–30.9). 12 had jaundice and 57 weight loss as a presenting feature. 58 presented with weight loss ≥15%. 25 babies had not stooled in the 24 h prior to admission. Serum sodium fell by median 12.9 mmol/l per 24 h (range 0–30). No baby died, had seizures or coma or was treated with dialysis or a central line. At discharge, babies had regained 11% of initial birth weight after a median admission of 5 (range 2–14) days. 10 were exclusively breast fed on discharge from hospital.

Conclusions Neonatal hypernatraemia at this level, in this population, is strongly associated with weight loss. It occurs almost exclusively after attempts to initiate breast feeding, occurs uncommonly and does not appear to be associated with serious short term morbidities, beyond admission to hospital.