The authors used a nominal response item response theory model to estimate category boundary discrimination (CBD) parameters for items drawn from the Emotional Distress item pools (Depression, Anxiety, and Anger) developed in the Patient-Reported Outcomes Measurement Information Systems (PROMIS) project. For polytomous items with ordered response categories, CBD parameters index the degree to which a particular dichotomous distinction (e.g., a response in category two vs. one) discriminates trait levels. Findings indicated that 25 of the 86 PROMIS items displayed statistically significant (p [less than or equal to] 0.05) within-item variation in CBD parameters as judged by a Wald test. The most common finding was that the CBD parameter for the first ("never vs. rarely") and last ("sometimes vs. often/always") response distinction were higher than the CBD parameters for the remaining distinction ("rarely vs. sometimes"). The implications of significant CBD variation for model choice, scale analysis, and for scoring individual differences are reviewed. (Contains 4 tables and 7 figures.)