AIM: To evaluate the transition from a longitudinal to block/modular structure of preclinical courses in a medical school adapting to the process of higher education harmonization in Europe. ----- METHODS: Average grades and the exam pass rates were compared for 11 preclinical courses before and after the transition from the longitudinal (academic years 1999/2000 to 2001/2002) to block/modular curriculum (academic years 2002/2003 to 2004/2005) at Zagreb University School of Medicine, Croatia. Attitudes of teachers toward the 2 curriculum structures were assessed by a semantic differential scale, and the experiences during the transition were explored in focus groups of students and teachers. ----- RESULTS: With the introduction of the block/modular curriculum, average grades mostly increased, except in 3 major courses: Anatomy, Physiology, and Pathology. The proportion of students who passed the exams at first attempt decreased in most courses, but the proportion of students who successfully passed the exam by the end of the summer exam period increased. Teachers generally had more positive attitudes toward the longitudinal (median [C]+/-intequartile range [Q], 24+/-16) than block/modular curriculum (C+/-Q, 38+/-26) (P=0.001, Wilcoxon signed rank test). The qualitative inquiry indicated that the dissatisfaction of students and teachers with the block/modular preclinical curriculum was caused by perceived hasty introduction of the reform under pressure and without much adaptation of the teaching program and materials, which reflected negatively on the learning processes and outcomes. ----- CONCLUSION: Any significant alteration in the temporal structure of preclinical courses should be paralleled by a change in the content and teaching methodology, and carefully planned and executed in order to achieve better academic outcomes.