The thesis of this paper is that the growth of the field of medical informatics, while seemingly a potential threat to medical librarianship, is in fact an opportunity for librarianship to both extend its reach and also to further define its unique characteristics in contrast to those of medical informatics. Furthermore, because medical librarianship represents a sector of librarianship itself, the relationship between medical informatics and medical librarianship provides an example of the influence of a cognate field on the profession of librarianship that may extend across the profession. This paper defines both medical informatics and librarianship, their areas of overlap and their claims to professionalism. The "informationist" a new health professional, which was recently proposed in one of the leading U.S. medical journals, illustrates one model of collaboration between the two fields. The paper concludes by suggesting new educational pathways. (Contains 24 references.) (Author)