Twenty-four tenth-grade students were randomly assigned to three groups as part of a study of the effects of teacher comments on student essays. Throughout the course, one group received comments to indicate faults, one group received comments correcting errors, and one group received comments designed to foster thinking. At the beginning and end of the course, students wrote a sample essay and completed a short questionnaire to measure attitude toward writing. Essays were scored holistically and were also analyzed for spelling errors, agreement errors, capitalization errors, run-ons, fragments, vocabulary (measured by average word length), single word modifiers, subordinate clauses, and depth of subordination between sentences. Of twelve analyses of covariance, the only significant difference occurred for vocabulary; growth on this measure favored the group receiving comments designed to foster thinking. It was concluded that the different types of teacher comments produce about the same improvement in student writing ability. (AA)