Aim: The aim of the study consisted in content validation of the defining characteristics, and related factors of the nursing diagnosis of deficient fluid volume [dehydration] (00027), from the perspective of neonatal nurses. Design: Content validation study. Methods: The defining characteristics of the nursing diagnosis of deficient fluid volume [dehydration] (00027) were evaluated with the help of the Fehring Diagnostic Content Validity Model. The set consisted of 125 experts/nurses. The significance of the defining characteristics was rated by the nurses on a Likert scale from 1 to 5. Data collection was carried out from March to September 2016. Results: seven characteristics from 32 were designated primary defining characteristics by the experts: decreased urine output, sunken fontanelle, change in skin turgor, dry mucous membranes, increased urine concentration, elevated haematocrit, and general appearance of exhaustion. 25 characteristics were designated secondary defining characteristics; 12 of which were characteristics from NANDA – I. Items such as: constipation, decreased pulse volume, cramps, absence of tears, and abdominal distension achieved a weighted score under 0.50. All from the group of related factors were defined as secondary by the experts. Conclusion: The set of experts regarded the primary significant characteristics for the determination of the nursing diagnosis of deficient fluid volume [dehydration] in newborns to consist in seven defining characteristics. All related factors were regarded to be secondary.