Competition and complementarities between rms can occur simultaneously. To exibly identify the nature of strategic interaction between GPs and specialist types in Belgium, we present a sequential equilibrium entry game of incomplete information. The results indicate that some specialists are strategic complements (gynecologists, TNE-specialists) and some are strategic substitutes in the entry decision of GPs (dermatologists, ophthalmologists, psychiatrists). With free choice of physician, this is indicative for patientschoice behavior: for the latter group of specialists, patients often wrongly self-refer. Although introducing mandatory referral schemes would correct this ine¢ ciency, it would generate sustainability problems for the current body of specialists. Keywords: entry, strategic interaction, GPs, specialists, gatekeeping JEL-codes: I11, L10 Contact: firstname.lastname@example.org I thank Frank Verboven, Pedro Pita Barros, Hans Degryse, Michael Mazzeo, Erik Schokkaert, Patrick Van Cayseele, Xavier Boutin, Geert Dhaene, Stijn Ferrari and various seminar participants at CEPR Applied IO School 2008, 6th Annual IIOC, NIE 2008, EARIE 2008, Tilburg University, RWTH Aachen, KULeuven, Kellogg School of Management and Antwerp University for helpful comments. Furthermore, I am grateful for the hospitality of Kellogg School of Management at Northwestern University and to Dendrite International Belgium and RIZIV/INAMI for data provision. I am also grateful to the Flemish Science Foundation for nancial support (FWO, Grant G.0089.04) and to EARIE08 (Young Economist Essay Award 2008) and VVE (VVE-prijs 2009) for awarding this paper.