Following high school, he enrolled at Santa Rosa Junior College, where he competed on the swimming and track & field teams and graduated with academic honors.

[8] He returned to Stanford for a pathology residency in 1946 and then spent a year as a research fellow in cardiology at the Thorndike Memorial Laboratory at the Harvard Medical School in Boston, Massachusetts.

[15] During a trip to the Peruvian Andes in 1959, Hultgren encountered the condition of high altitude pulmonary edema (HAPE), which was relatively unknown at the time and yet to be described in U.S. medical literature.

[16] In 1961, he was the first American researcher to define the clinical effects of this altitude sickness, which he presented that year at the annual meeting of the Western Association of Physicians in Carmel, California.

[10] During the next 10 years, he made many trips to the Chulec General Hospital in the Peruvian Andes city of La Oroya (12,300 ft) to study HAPE, chronicling his findings in multiple U.S. academic medical publications.

[15] Following his Andes excursions, Hultgren continued his study of high altitude illnesses at the White Mountain Barcroft Research Lab in the Sierra Nevada, on the peaks around Leadville, Colorado, and at the Mount Everest Base Camp in Nepal.

[7][21] He was also a lifelong member of the Sierra Club and California Academy of Sciences, an advocate and supporter of the Sempervirens Fund, and an instructor for Mountain Travel Medical Seminars in Alaska, Patagonia, and Nepal.

In 1990, Hultgren received the Albion Walter Hewlett Award, an honor bestowed by his fellow Stanford Medical School faculty members for "the physician of care and skill who has committed to discovering and using biologic knowledge, wisdom, and compassion to return patients to productive lives.

[6][11][15] In 2003, Hultgren's wife, Barbara, donated his research papers to the Mandeville Special Collections Library at the University of California, San Diego (UCSD), where they are available to the medical community.

[14] As an ardent student of natural history, he taught himself and learned the names of every common native Sierra Nevada species of mammal, reptile, bird, fish, tree, plant, and wildflower.

Hultgren was so captivated by the Sierra that he considered joining the U.S. Forest Service following graduation from Stanford, before ultimately deciding to pursue a career in medicine.

Among his favorite pursuits was an annual two-week Boy Scout troop backpack trip, crossing the Sierra Nevada from east to west and traversing sections of the John Muir Trail.