This study analyses the effect of spatial concentration of general hospitals, the appearance of independent treatment centers (in Dutch: Zelfstandige Behandelcentra: ZBCs) and the concentration of health insurers on production volume and costs since the introduction of market-oriented health care reforms in the Netherlands. We use regression analyses of 1,345,144 patient-level hospital data for fifteen major diagnosis treatment combinations (in Dutch: Diagnose Behandeling Combinaties: DBCs), representing 70% of the managed competition segment (the so-called B-segment).We find that spatial concentration of hospitals and concentration of insurers do not affect health care production volume. More competitive hospital markets are associated with higher cost of most DBCs studied. Surprisingly, hospitals operating under insurers with high monopsonic power incur higher average DBC-cost than hospitals operating under insurers with more dispersed power. The number of independent treatment centers in the hospital’s vicinity is positively related to health care volume and average cost.