The following questions are addressed: (1) Are there sex differences in achievement, either in entering knowledge of geometry in the fall, or in achievement in acquiring standard geometry content by year's end? (2) Are there sex differences in the performance of students on the van Hiele test, either at the beginning or end of the year? and (3) Are sex differences exhibited in the writing of geometry proofs? Subjects were 1392 boys and 1307 girls in 99 classes, grades seven through twelve, from 13 schools located in 5 states. Schools were chosen to include a variety of regions and socioeconomic groups, and the population included substantial black, Hispanic, and Oriental minorities. A proof-writing sample consisted of 1520 students in 74 classes which had studied proof and whose teachers agreed to participate. Results indicated that boys were superior in content knowledge upon entering the geometry courses. Adjusting scores for this entering knowledge led to observation that girls were equally able to learn both geometry content and exhibit higher cognitive reasoning required in writing geometry proofs. Therefore, no sex differences in geometry achievement were found. (MP)