"The Critical Word Factor," based on word recognition demands of texts, is a measure of text difficulty designed specifically for texts used by beginning readers. The measure is a function of the number of new, unique words per 100 running words of text that fall outside a designated curriculum. The authors investigated the validity of the Critical Word Factor from data on beginning readers' speed, accuracy, and comprehension after students read texts with different numbers of critical words. Analyses of variance indicated significant main effects for the Critical Word Factor on reading speed, accuracy, and comprehension. Mean differences on the three variables were in the predicted direction; results for speed and accuracy were stronger than were those for comprehension. Additional analyses showed that words predicted by the model to be hard were hard, and those predicted to be easy were easy. (Contains 3 tables.)