The authors present an economic evaluation performed alongside a randomized controlled trial of mentalization-based treatment in a day hospital setting (MBT-DH) versus specialist treatment as usual (S-TAU) for borderline personality disorder (BPD) with a 36-month follow-up period. Ninety-five patients from two Dutch treatment institutes were randomly assigned. Societal costs were compared with the proportion of BPD remissions and quality-adjusted life years (QALYs) measured using the five-dimensional EuroQol instrument. The incremental societal costs for one additional QALY could not be calculated. The costs for one additional BPD remission with MBT-DH are approximately €29, 000. There was a 58% likelihood that MBT-DH leads to more remitted patients at additional costs compared with S-TAU, and a 35% likelihood that MBT-DH leads to more remissions at lower costs. MBT-DH is not cost-effective compared with S-TAU with QALYs as the outcome, and slightly more cost-effective than S-TAU at 36 months with BPD symptoms as the outcome.