 He began studying at Johns Hopkins University the next year, before traveling to Germany and studying at Johannes Orth's laboratory at the University of Göttingen in 1893.From 1893 to 1896, Cullen was in charge of gynecological pathology at Johns Hopkins, and in 1919 he was named a professor of clinical gynecology.Cullen researched gynecological diseases including uterine cancer and ectopic pregnancy and promoted extensive use of diagrams in biomedical publishing.