The Global Improvement Scale (GIS) assesses multiple irritable bowel syndrome (IBS) symptoms using a patient-defined 7-point Likert scale ranging from symptoms substantially worse to substantially improved. To evaluate the scale as an efficacy end point, data were collected from two 12-week randomized, double-blind, placebo-controlled studies of female nonconstipated IBS patients. GIS responders were defined as having substantial or moderate improvement in IBS symptoms. GIS responders had more days with satisfactory control of urgency, firmer stools, fewer stools per day, and fewer days with incomplete evacuation compared to nonresponders. Substantially more GIS responders (90% and 89% in studies 1 and 2, respectively) were satisfied or very satisfied with their treatment overall compared to nonresponders (13% and 11%) (r = 0.8 in both studies). GIS responders had greater satisfaction with medication relief of pain and discomfort and the time needed to return to usual activities. Favorable correlations between GIS and work and nonwork productivity losses were observed. Correlation of the GIS measure with IBS clinical end points establishes the validity of the GIS for measuring improvement in IBS symptoms. The GIS may be useful in assessing the efficacy of IBS interventions in future clinical trials.