Purpose: Reliability and validity are important measures of the quality of a research or clinical instrument. This research determined the inter-rater and test-retest reliability of a stool consistency classification system and the agreement between the subjects' and expert investigators' classifications. Methods: Two studies were conducted with adult volunteers. Stool consistency was described by words only and words + diagrams. To determine inter-rater reliability subjects in each of 3 groups (20 nurses, 20 nursing students, and 20 lay persons) classified the consistency of 12 stool specimens. To determine test-retest reliability, 43 additional subjects classified the consistency of 9 stool specimens in 2 sessions. Outcome measures: Outcome measures were reported per individual stool specimen and for all stool specimens. The outcome measures were the consistency classifications by the 3 groups of raters when the 2 types of descriptors (word only and word + diagram) were used. Other outcomes were the consistency classifications of the subjects on the 2 days of stool evaluation and the classifications of the subjects compared with those of the investigators. Results: No significant difference was found among the stool consistency classifications among nurses, nursing students, or lay persons. Classifications were similar for 11 of 12 stool specimens when either word-only or word + diagram descriptions were used. No significant difference was found among the classifications between days 1 and 2. At least 75% of the subjects' classifications of stool consistency in both studies agreed highly with those of the investigators. Conclusions: The reliability and validity of the stool consistency classification system are good. The word-only descriptions of the consistency classifications appeared to be equally as effective as the word + diagram descriptions. (J WOCN 2001;28:305-13.).