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Objective: The impact factor has emerged as the most popular index of scientific journals’ resonance. In this study we aimed to examine the impact factor trends of journals published by scientific bodies in the United States of America (USA) and Europe (EU). Methods: We randomly chose 11 categories of Journal of Citation Reports and created three research classes: clinical medicine, laboratory medicine, and basic science. The impact factor values for the years 1999–2015 were abstracted, and the impact factor of US and EU journals was studied through the years. Results: A total of 265 journals were included in the final analysis. The impact factor of US journals was higher than that of EU journals throughout the study period. In addition, for both US and EU journals the median impact factor increased throughout the study period. The rate of annual change in the impact factor throughout the study period was lower for US than EU journals (1.85% versus 3.55%, P=0.019). A higher median annual increase was seen in the impact factor during the period 1999–2008 compared to the period 2009–2015 for both US (P