A multicenter study involving 204 adults with relapsing-remitting multiple sclerosis (RRMS) assessed the dimensionality and item characteristics of the Mishel-Uncertainty of Illness Scale (MUIS), a generic self-assessment tool. Mokken analysis identified two dimensions in the MUIS with an appropriate item and overall scale scalability after excluding nonclassifiable items. A refined 12-item MUIS, employing a grade response model, effectively discriminated uncertainty levels among RRMS patients (likelihood ratio test p-value = .03). These findings suggest the potential value of the 12-item MUIS as a reliable measure for assessing uncertainty associated with the course of illness in RRMS.