Cullen researched gynecological diseases including uterine cancer and ectopic pregnancy and promoted extensive use of diagrams in biomedical publishing. Cullen's sign, a discoloration of the skin about the navel which is regarded as a sign of a ruptured ectopic pregnancy, is named for him. He wrote alone, and in collaboration, four important monographs:
^ abScottish Rite (Masonic order). Supreme Council for the Southern Jurisdiction (1953). The New age magazine. Supreme Council, 33,̊ Ancient and Accepted Scottish Rite of Freemasonry of the Southern Jurisdiction, U.S.A. p. 256. Retrieved 12 March 2012.