Early in 1974, Harry Soroff, the chair of surgery at SUNY Stony Brook, recruited Dennis to the recently-opened medical school's faculty. The offer included generous research space and clinical facilities, guaranteed support for research, and no administrative duties. Though concerned about the newness of the Stony Brook program, and about recovering his rusty surgical skills, Dennis was eager to leave NIH, and returned to New York in 1975. Once again he found himself helping to build a surgical program and a coordinated system of affiliated hospitals at a relatively new institution. However, he was able to devote much of his time to teaching and to clinical research, working mainly at the Northport Veteran's Administration Hospital. There he returned to several of his earlier interests including gastrointestinal surgery and wound-healing studies.

Dennis retired from Stony Brook in 1988, and moved back to St. Paul, Minnesota. He came out of retirement in 1991 to take over as director of the Cancer Detection Center (CDC) at the University of Minnesota. Owen Wangensteen had founded the CDC in 1948, as a non-profit, public service project of the department of surgery. Many of the surgeries he and his staff performed at the time involved cancer, and they knew they could save more patients if they caught the cancers earlier. CDC clients could, for a modest fee, get an annual comprehensive screening examination and if needed, referrals for treatment. It was an innovative program for many years, but when Dennis was recruited, the CDC was in decline and the department of surgery had considered closing it after the previous director's death. As he had throughout his career, Dennis applied his knowledge, experience, and characteristic patience to sorting out yet another tangled situation.

The CDC had many problems. The clinic space had been split by new construction at the university, and was deteriorating. The staff was having difficulties coordinating with the hospital's radiology and other diagnostic services, and follow-up on positive diagnostic findings was slow. The program's income was declining because it had lost many of its subscribers after raising the exam fee several years earlier. And there were other issues, such as relations with non-university physicians whose patients were screened at the CDC. Dennis spent several years negotiating with university officials for new space for the center. Meanwhile, he reorganized the center's procedures, and arranged better coordination with hospital labs, and improved follow-up protocols. He and the staff brainstormed ways to recruit more clients, including working with local employers who might subsidize the exam fees for their workers. They also looked into getting insurance plans to cover their screening service. Just as the CDC was finally moved into new space in 1993, however, the department of surgery found its income reduced when its experimental anti-lymphocyte globulin (ALG) production was shut down. The immune suppressant had been developed by the department chair, John Najarian, in 1969, to prevent initial rejection in kidney transplants; the FDA had licensed it as an investigational new drug, and Najarian's lab produced and sold ALG to transplant surgeons all over the country, reinvesting the profits in the medical school. This turned out to be against FDA regulations, and in 1992 the ALG program was shut down. The income--over $3,000,000 per year--had allowed the medical school to prosper despite state budget cuts, but now that cushion was gone. Dennis faced the task of seeking outside funding for the CDC, and appealed to Minnesota state legislators for at least short-term aid. He was able to keep the center open until 1996, when it was absorbed into the university hospital's primary care and general medicine division. At that point, citing his age (86) and the limits imposed by his increasing macular degeneration, Dennis retired for the third and last time.

During retirement Dennis continued to attend weekly surgical rounds at the University of Minnesota Hospital and the Veterans Administration Hospital, as well as professional conferences. He and his wife Mary also spent much of their time at their lakeside house in Cable, Wisconsin, where the Dennis family had vacationed since his childhood. Dennis loved fishing there, and was often joined by family and friends. He worked on several inventions during these years, including devices to augment his deteriorating vision, a gadget that allowed him to steer his fishing boat with his feet, and a bread-slicing frame.

In his last years he developed dementia, and died from complications of that disease on July 11, 2005, at the age of 96. The legacy of this quiet, innovative surgeon-inventor and educator included not just his heart-lung machine and other devices, but the hundreds of surgeons he trained, who have continued to expand the frontiers of cardiac surgery and cardiac assistance.