A previous paper reported derivation of the Crohn's Disease Activity Index (CDAI) from data on 112 patients. We have now rederived the values of the eight coefficients of the CDAI using data from 1058 visits of patients enrolled in the National Cooperative Crohn's Disease Study and the Trial of Adjunctive Sulfasalazine in Crohn's Disease. The rederived coefficients are similar to the original ones. New and original index values calculated on the same data from patient visits correlated very highly. Because there is so little difference between the original and the rederived versions, continued use of the original version is suggested in order to maintain comparability of new studies employing this tool with those previously accomplished.