Abraham Flexner persuaded the medical establishment of his time that teaching the sciences, from basic to clinical, should be a critical component of the medical student curriculum, thus giving rise to the "preclinical curriculum." However, students' retention of basic science material after the preclinical years is generally poor. The authors believe that revisiting the basic sciences in the fourth year can enhance understanding of clinical medicine and further students' understanding of how the two fields integrate. With this in mind, a return to the basic sciences during the fourth year of medical school may be highly beneficial. The purpose of this article is to (1) discuss efforts to integrate basic science into the clinical years of medical student education throughout the United States and Canada, and (2) describe the highly developed fourth-year basic science integration program at the University of Pittsburgh School of Medicine. In their critical review of medical school curricula of 126 U.S. and 17 Canadian medical schools, the authors found that only 19% of U.S. medical schools and 24% of Canadian medical schools require basic science courses or experiences during the clinical years, a minor increase compared with 1985. Curricular methods ranged from simple lectures to integrated case studies with hands-on laboratory experience. The authors hope to advance the national discussion about the need to more fully integrate basic science teaching throughout all four years of the medical student curriculum by placing a curricular innovation in the context of similar efforts by other U.S. and Canadian medical schools.