This paper addresses the question of how grammar checkers may help or hinder students by analyzing the performance of the Microsoft Word 97's Grammar Checker at flagging and explaining errors frequently made by college students. Thousands of sentences were fed into the program. Results indicate that it caught some of the errors reliably (e.g. subject-verb agreement errors) and others at least occasionally (e.g. comma and capitalization errors). However, the program does not catch most pronoun or modifier errors, and it occasionally labels a correct sentence as an error. An indication is provided regarding the writing style settings needed to catch each type of error. Findings suggest that some students can improve their documents using Grammar Checker if their writing is relatively free of errors, and if they are willing to consult a grammar handbook occasionally. Recommendations to teachers promoting Grammar Checker include: (1) encouraging students to proofread; (2) ensuring that students have a basic understanding of sentence structure and grammar terminology; (3) explaining limitations of Grammar Checker; (4) suggesting that students select "custom" as the writing style; (5) helping students select grammar and style settings appropriate for their level of expertise; (6) encouraging students to run Grammar Checker with Help turned on; and (7) suggesting that students consult a grammar text when they do not understand information in the Help box. Contains 31 figures presenting Help boxes generated by the software program. (EF)