Web-based computer-aided instruction (CAI) has become increasingly important to medical curricula. This multi-year study investigated the effectiveness of CAI and the factors affecting level of individual use. Three CAI were tested that differed in specificity of applicability to the curriculum and in the level of student interaction with the CAI. Student personality preferences and learning styles were measured using the Meyers Briggs Type Indicator (MBTI) and Kolb's Learning Style Inventory (LSI). Information on "computer literacy" and use of CAI was collected from student surveys. Server logs were used to quantify individual use of respective CAI. There was considerable variability in the level of utilization of each CAI by individual students. Individual use of each CAI differed and was associated with gender, MBTI preferences and learning style, but not with "computer literacy." The majority of students found the CAI useful for learning and used the CAI by themselves. Students who accessed the CAI resources most frequently scored significantly higher on exams compared with students who never accessed the resources. Our results show that medical students do not uniformly use CAI developed for their curriculum and this variability is associated with various attributes of individual students. Our data also provide evidence of the importance of understanding student preferences and learning styles when implementing CAI into the curriculum. (Contains 5 tables.)