Competition and complementarities between firms can occur simultaneously. To flexibly 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 patients’ choice behavior: for the latter group of specialists, patients often wrongly self-refer. Although introducing mandatory referral schemes would correct this inefficiency, it would generate sustainability problems for the current body of specialists.