This paper describes one college-level English-as-a-Second-Language teacher's use of error analysis in an effort to understand students' problems with reading comprehension and writing. The research was undertaken in a Japanese junior college. Three studies are presented. In the first, 59 students in a general English course listed sentences they found difficult in a textbook, and why they were prevented from understanding them. In the second, 85 students in an English word processing course read authentic materials (e.g., newspapers, World Wide Web pages) and recorded, in a similar manner, the sentences they found difficult to understand. In the third study, electronic mail messages to the teacher from the same group of 85 students were analyzed for errors. Nine categories of error were identified: syntactic errors in constructions that are different in Japanese and English; syntactic errors in constructions due to failure in sentence processing; lexical errors occurring mostly with polysemy; lexical errors in interpreting word classes; lexical errors involving misinterpretation within a word class; lexical errors involving cultural misunderstanding; lexical errors caused by formal similarity of words; unknown common/colloquial expressions in English; and errors in the text. Comprehension and productive errors are compared. Contains 9 references. (MSE)