As the use of nursing diagnoses in clinical practice increases, systematic research is necessary to appropriately validate them. Validity studies could be expedited by the use of information technology and computerized clinical data bases. The purpose of the following descriptive study was to validate the defining characteristics (risk factors for potential diagnoses) of the four nursing diagnoses related to alterations in fluid volume proposed by the North American Nursing Diagnosis Association using a nursing minimum data set (NMDS) collected from a computerized nursing data base. A sensitivity measure, the proportion of subjects with specific defining characteristics for a given diagnosis among the total number of subjects with the respective diagnosis, was used to estimate validity. The study was conducted in a 265-bed, mid-western, community hospital in which computerized nursing diagnosis care planning is used. The elements of the NMDS and defining characteristics were collected from a systematic sample (N = 191). The sample consisted of a proportion of about half of computerized discharge patient care plan summaries from each of the four diagnostic labels related to alteration in fluid volume obtained from medical records during the 1987 calendar year. Discharge patient care plan summaries contained at least one of four nursing diagnoses related to alteration in fluid volume. The results show the existence of individual defining characteristics as well as combinations of defining characteristics meeting validation criteria for each diagnostic label. Never-documented defining characteristics also were identified.