Three-, four-, and five-year-old children's categorical and comparative understanding of high and low were examined in two experiments. Categorical knowledge was assessed by presenting subjects with a single object at varying heights (from 0 to five feet above the ground), and asking if the object was high or low. Comparative understanding of the terms was assessed by showing children two objects at a time and asking which was higher or lower. We observed two patterns of performance in children's categorical treatments: younger children in particular defined disjoint categories for high and low such that they only labelled the extreme heights as high or low, and maintained that middle heights were neither high nor low. Older children defined either-or categories such that all heights were labelled either high or low. We also found that children who defined either-or categories made correct comparative judgments across the entire range of variation whereas children who defined disjoint categories could only judge which of two objects was higher if the objects were not low (at 0 and 1 feet) and which of the objects was lower if the objects were not high (at 4 and 5 feet). The results were interpreted as reflecting a lack of appreciation that the terms are interdefined as negations of each other, and were discussed in terms of the similar semantic-congruity effects found in adults.