Children aged from 3.5 to 5.5 years were tested on their comprehension of the terms taller, shorter, more, and less in five types of sentences: truncated, explicit standard, expanded explicit standard, two-dimensional second-clause subject noun, and two-dimensional second-clause subject pronoun. Many childen performed poorly on less and shorter in truncated sentences; the presence of explicit comparative cues did not improve these children's performance. Children who performed perfectly on truncated sentences still had difficulty understanding two-dimensional comparisons, especially those with a second-clause subject pronoun. These children interpreted only the first clause of the second-clause subject pronoun sentences. The results suggest that many children are capable of understanding two-dimensional comparisons, but perform poorly on the second-clause subject pronoun sentences because of uncertainty about the referent of the pronoun.