The aim of this study is to identify and analyse the most important reasons that influence absenteeism in university classrooms from the students' perspective. Data have been gathered through a questionnaire answered by 1,896 students from the Business Administration, Economics and Sociology degrees of the Universitat de Barcelona, Spain. Both a difference of means and exploratory factor analyses have been applied. Findings show that students differ on these reasons regarding their year and degree of study. The results of the factor analysis reveal five dimensions of reasons for absenteeism: (1) students' own planning, (2) teaching methodology, (3) learning methodology, (4) course characteristics, and (5) external sources available. These factors call for effective action lines that eliminate the inefficiencies of public resources.