Guidance on the prevention of hyponatraemia in children was issued by DHSSPSNI in March 2002. Two years later Dr Henrietta Campbell, the Chief Medical Officer, wrote to the Chief Executives of acute and combined trusts to seek assurances that the guideline had been incorporated into clinical practice and its implementation monitored. This paper reports the findings of the first prospective study undertaken to examine practice following introduction of the guidance. The evidence suggests that implementation has so far been incomplete and highlights problem areas. The paper reflects on potential explanations for the findings and makes practical suggestions for improvement.