A number of models of young adult cognitive development postulate stage characteristics. However, with few exceptions, only weak stage-related claims have been tested. The primary purpose of this study was to evaluate the stage properties of the Reflective Judgment model using six-year longitudinal data on three groups of young adults. The secondary purpose was to examine cohort effects on Reflective Judgment scores. Kitchener and King's (1981) Reflective Judgment Interview (RJI) was used as a measure of Reflective Judgment. RJI mean scores increased significantly (p < .01), Davison's test of sequentiality (1979) supported the sequence of the model, stage reversals could be attributed to error, and, using a conservative definition, stage skipping occured in only 14% of the cases. The overall correlation between RJI score and age was .79. Subjects' modal score was consistent across problems 75% of the time. A cohort effect was also found. The data support the sequentiality of the Reflective Judgment stages and the consistency of subjects across problems.