Background: Medical costs of (psychiatric) illness can be validly measured with patient report questionnaires. These questionnaires comprise many detailed items resulting in lengthy administrations. Objectives: We set out to find the minimal number of items needed to retrieve 80% and 90% of the costs as measured by the Treatment Inventory of Costs in Patients with psychiatric disorders (TIC-P). Methods: The TIC-P is a validated patient-reported outcome measure concerning the utilization of medical care and productivity losses. The present study focused on direct medical costs. We applied data of 7756 TIC-P administrations from three studies in patients with mental health care issues. Items that contribute least to the total cost were eliminated, providing that 80% and 90% of the total cost was retained. Results: Average medical costs per patient were 658 over the last 4 weeks. The distribution of cost was highly skewed, and 5 of the 14 items of the TIC-P accounted for less than 10% of the total costs. The 80% Mini version of the TIC-P required five items: ambulatory services, private practice, day care, general hospital, and psychiatric clinic. The TIC-P Midi 90% inventory required eight items. Both had variance between the three samples in the optimal choice of the items. Conclusions: The number of items of the TIC-P can be reduced considerably while maintaining 80% and 90% of the medical costs estimated by the complete TIC-P. The reduced length makes the questionnaire more suitable for routine outcome monitoring.