The modified American Shoulder and Elbow Surgeon's (M-ASES) questionnaire is purported to be a non-region specific functional measure of the entire upper extremity. The purpose of this study was to determine the factor structure of the M-ASES and to determine item-fit of the M-ASES using item response theory (IRT). Analyses included univariate baseline demographics, factor analysis, convergent/construct validation with the SF-12, and graded response IRT of the M-ASES. 964 patients of an orthopedic practice with a variety of upper extremity dysfunctions participated in this trial. The M-ASES demonstrated two dimensions (wrist/hand and shoulder dysfunction) and exhibited excellent discrimination and threshold specification. The instrument correlated well with the mental and physical dimensions of the SF-12. The M-ASES should be considered an excellent tool for measure of whole upper extremity dysfunction.