Dr. McCallum was recruited to the University of Virginia in September 1985 to become the Paul Janssen Professor of Medicine, and chief of the division of gastroenterology, hepatology, and nutrition for ten years. He then accepted the position of professor of medicine, chief of gastroenterology and hepatology at Kansas University Medical Center where he was also director of the Center for GI Nerve & Muscle Function & GI Motility. He was elected to the American Society for Clinical Investigation, and is also a fellow of the American College of Physicians, the Royal Australasian College of Physicians, and the American College of Gastroenterology.

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Dr. McCallum has published more than 390 articles in peer-reviewed scientific journals, 100 textbook chapters, and has edited 13 scientific textbooks. The most recent textbooks that he has co-edited are entitled "GI Motility Testing – A Laboratory and Office Handbook," a reference text in this field and "Gastroparesis: Pathophysiology, Presentations, and Treatment." He serves on the editorial boards of eight scientific publications and committees of the major gastroenterology associations in this country. He was president of the Southern Society for Clinical Investigation, International Electrogastrogram Society, Trustee of the American College of Gastroenterology, and received the Southern Medical Association Award for Original Research. Pharmaceutically supported research is conducted by his research team and original research has been supported with VA Merit Reviewing funding, Pharmaceutical, American Diabetes Association, Foundation Grants, Small Business NIH Awards and most recently NIH Grant funding as part of the NIH Consortium in Gastroparesis.

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Dr. McCallum's research has been focused on the physiology, pathophysiology, and pharmacology of gastrointestinal smooth muscle relating to disorders of motility and functional bowel disease involving the esophagus, stomach, small bowel, and colon. He has been continuously funded by the NIH, American Diabetes Association, and industry grants. He has made major advances in diagnosis and treatment of gastroparesis, especially in diabetes. As founding chair of medicine, he built a nationally competitive department of medicine at TTUHSC El Paso, as well as advancing research funding and starting fellowship programs. For the last 20 years his pioneering efforts in the area of electrophysiology of the gut and, specifically, electrical pacing of the stomach resulted in new therapeutic concepts for nausea and vomiting. Other research contributions that continue are in the recognition of cyclic vomiting as a major new etiology of nausea and vomiting in adults, diagnosing and treating dumping syndrome and rumination syndrome as well as other gut motility problems, such as anorectal motility disorders and irritable bowel and the spectrum of small bowel bacterial overgrowth. Developing new pharmacologic and medical device therapies for these entities are also ongoing projects.