We assessed carpal collapse by measuring the capitate-radius (CR) distance on standard plain radiographs. This new method required validation of diagnostic accuracy, so we compared it with the method of Nattrass et al known as revised carpal height (RCH). We studied wrist radiographs from 16 normal subjects and 11 patients with unilateral Kienböck's disease. We found that there was a significant difference in the left/right CR index between the normal wrists and those with Kienbock's disease (p < 0.001). The use of left/right RCH index showed no significant difference (p = 0.30). Diagnostic accuracy was shown to be higher for the CR index using ROC curves. We then assessed 40 normal wrists and found the mean CR index to be 0.999 +/- 0.034, and suggest that values less than 0.92 are abnormal. The CR index can be used for diagnosis in unilateral carpal collapse, and for monitoring progress where the condition is bilateral.