During his medical studies at Johns Hopkins, Green met John Money, who was an assistant professor there, and started collaborating with him on research, initially on boys displaying substantial cross-gender behavior. In Money's obituary, Green acknowledges Money and Robert Stoller, as well as his father, Leo H. Green, for having set the course for his life and career. In the mid-1960s, Money introduced Green to Harry Benjamin, whom Green acknowledges as having "further honed" his career. In 1969 Green and Money co-edited "Transsexualism and Sex Reassignment", published by Johns Hopkins Press.