The purpose of this study was to investigate (a) the role of a yearlong geometry course on high school geometry students' logical thinking and proof construction abilities, (b) the linkage between students' logical thinking and proof construction abilities, and (c) the impact of dynamic geometry software on students' performance. In addition, this study also ventured to determine if the type of geometry course had any impact on students' logical thinking and proof construction achievement. The sample for the study consisted of 1,325 high school geometry students enrolled in regular, honors, and mastery courses in four high schools from the school district affiliated with the Local Education Agency (LEA) during the academic year 2004-2005. Geometer's Sketchpadä (GSP) was assumed to represent the dynamic geometry software. Responses of students on two pre-tests and two post-tests, each with one on logical thinking and one on proof, were analyzed to address the research questions. Results of the analyses indicated no significant effect of the yearlong geometry course on the performance of students on proof tests but a fairly significant effect on the tests of logical thinking. Use of GSP was found to have some impact on honors and mastery students' performance on proof post-tests. Honors students showed a higher logical thinking level than their regular and mastery counterparts in both GSP and non-GSP groups. There was a significant positive correlation between students' performance on the tests of logical thinking and proof.