An efficient and precise measure of balance is needed to improve administration efficiency and to reduce the assessment burden for patients. The purpose of this study was to develop a computerized adaptive testing (CAT) system for assessing balance function in an efficient, reliable, and valid fashion in patients with stroke. Two cross-sectional prospective studies were conducted. This study was conducted in the departments of physical medicine and rehabilitation in 6 hospitals. The participants were inpatients and outpatients who were receiving rehabilitation. A balance item pool (41 items) was developed on the basis of predefined balance concepts, expert opinions, and field testing. The items were administered by 5 raters to 764 patients. An item response theory model was fit to the data, and the item parameters were estimated. A simulation study was used to determine the performance (eg, reliability, efficiency) of the Balance CAT. The Balance CAT and the Berg Balance Scale (BBS) then were tested on another independent sample of 56 patients to determine the concurrent validity and time needed for administration. Seven items did not meet the model's expectations and were excluded from further analysis. The remaining 34 items formed the item bank of the Balance CAT. Two stopping rules (ie, reliability coefficient >0.9 or ≤6 items) were set for the CAT. The simulation study showed that the patients' balance scores estimated by the CAT had an average reliability value of .94. The scores obtained from the CAT were closely associated with those of the full item set (Pearson r=.98). The scores of the Balance CAT were highly correlated with those of the BBS (Pearson r=.88). The average time needed to administer the Balance CAT (83 seconds) was only 18% of that of the BBS. The convenience sampling of both samples may limit the generalization of the results. Further psychometric investigation of the Balance CAT is needed. The results provide strong evidence that the Balance CAT is efficient and has reliability and validity for patients with stroke. Copyright © 2010 American Physical Therapy Association.