The purpose of this study is to explore the impact and relative importance of course content and the role of the instructor on measures of effectiveness for on-line courses. Items from a standard student evaluation questionnaire are used to form measures of the quality of course content, the instructor's role in facilitating the course, and a number of 'global' effectiveness measures. Results from a number of stepwise regression analyses suggest that the impact and importance of these two dimensions of teaching effectiveness vary depending on the specific global effectiveness measure being assessed. Generally, both can be seen as strong predictors of global effectiveness of on-line courses, explaining from 0.569 to 0.743 of the variance in different global effectiveness measures. Further, the study identifies those standard questions used to form the course content and instructor's role indices that are of greatest importance in predicting global teaching effectiveness of on-line courses.Recent years have seen a tremendous increase in university courses taught over the Internet. Clark (1999) reported that over two-thirds of the 3,200 accredited colleges and graduate schools in the U.S. offer such courses. In 1999, Fornnaciari & Forte reported that 40 accredited U.S. business schools offered complete MBA programmes through distance education. More recently, U.S. News and World Report surveyed 2,000 institutions and noted their top 25 on-line programmes in the U.S. Although on-line education is here to stay, a key question is whether this new form of teaching can offer a quality learning experience. In describing the development of on-line courses and programmes, Marchese (2000) stated that "the action today, and most of the press coverage, goes to much larger-scale endeavours-institutional and corporate based-often for profit, aimed at capturing market share and creating new revenue streams. There is lots of striving and 'reach' out there, but little attention to 'richness' and learning itself"(p.4).This study seeks to determine the importance of two dimensions in the delivery of high quality on-line MBA courses: the content of the course and the role of the instructor. The research question addressed, as suggested by the title of this article is, "which is most important in achieving effectiveness in on-line courses, the instructor or the content of the course being offered?" Answering this question is important for a number of reasons. First, many