We appreciate all the kind remarks and suggestions from the many medical students during the last 5 years. Your positive reception has been an incredible encouragement, especially in light of the short life of the Case Files series. In this third edition of Case Files: Biochemistry, the basic format of the book has been retained. Improvements were made in updating many of the chapters. New cases include adenosine deaminase deficiency, maple syrup urine disease, alcoholic ketoacidosis, glucose-6-phosphate dehydrogenase deficiency, medium chain fatty acyl dehydrogenase deficiency, pheochromocytoma, and pediatric methomoglobinemia. We reviewed the clinical scenarios with the intent of improving them; however, their "real-life" presentations patterned after actual clinical experience were accurate and instructive. The multiple-choice questions have been carefully reviewed and rewritten to ensure that they comply with the National Board and USMLE format. Through this third edition, we hope that the reader will continue to enjoy learn diagnosis and management through the simulated clinical cases. It certainly is a privilege to be teachers for so many students, and it is with humility that we present this edition.