The reliability of short versus long passages in informal reading inventories was compared. After 132 fourth grade students were tested with the Standard Reading Inventory, Form B, they read longer passages in either the oral or silent modes. Students with higher silent reading scores read long passages orally, while students with higher oral reading scores or comparable oral/silent reading scores read long passages in the silent mode. The results suggested that the 70% criterion normally used with short passages inadequately predicted reading levels for the longer selections. The increased difficulty of long passages was evident by the marked decreases in comprehension scores. Students with better comprehension scores in either the short-silent or short-oral modes appeared to be frustrated by the longer reading materials. The students with comparable oral/silent reading levels in the short passages showed significantly different scores on the long passages, suggesting that the increased difficulty of long passages inhibited their comprehension considerably. Since the long passages appeared to be more difficult than short passages at the same levels, it was suggested that the trend toward using more lenient interpretation criteria in informal reading inventories might be based on false assumptions. (RL)