Objective The objective of the study was to evaluate the cost-effectiveness of implementing the Grip on Challenging Behaviour care programme (GRIP) on dementia special care units in comparison with usual care. Methods A stepped wedge design was used. Challenging behaviour and quality of life were measured using the Cohen Mansfield Agitation Inventory (CMAI) and the QUALIDEM. Quality-adjusted life years (QALYs) were calculated using the EuroQol-5D. Psychoactive medication use (range 0-5 per measurement) and sick leave were registered. Costs included medication, time spent on challenging behaviour and education. Costs and effects were analysed using linear multilevel regression. Incremental cost-effectiveness ratios were calculated. Statistical uncertainty was estimated using bootstrapping. Results Seventeen dementia special care units participated. GRIP led to improvement on the QUALIDEM subscale social relations (1.6; 95% CI 0.18 to 3.4) and on the use of psychoactive medication (-0.73; 95% CI -1.1 to -0.46) and to a decrease in QALYs (-0.02; 95% CI -0.06 to -0.003). No significant effects on CMAI, sick leave and other QUALIDEM subscales were found. The intervention was not cost-effective in comparison with usual care with regard to CMAI score, QALYs and sick leave. The willingness to pay should be 320€/point improvement on the QUALIDEM subscale social relations and 370€/psychoactive medication less to reach a 0.95 probability of cost-effectiveness. Conclusion It depends on how much society is willing to pay whether GRIP can be considered cost-effective. Because the appropriateness of the current methods for analysing cost-effectiveness in this specific population is uncertain, the positive effects on behaviour, medication and job satisfactions should also be taken in account in the decision making.