BACKGROUND: No apt method has been available to assess and monitor the responsiveness of services in meeting ongoing needs of patients with long-term mental illness. The present study examines the utility of a new metric for such a purpose, the Met Needs Index (MNI), applied to the Camberwell Assessment of Need (CAN). METHODS: The MNI was estimated as an aggregated measure of met need or beneficial outcome, based on annual staff rated CAN-assessments of 321 outpatients (76% psychotic disorders) in psychiatric care during 7 years. Corresponding confidence intervals were estimated with the bootstrap percentile method. RESULTS: The overall MNI was estimated at 0.71 (95% CI 0.69-0.74), indicating that identified needs in general were met during 71% of the intervals between the annual assessments. However, the MNI for specific need domains of the CAN ranged from 0.89 (95% CI 0.84-0.93) for 'food' to 0.11 (95% CI 0.07-0.16) for 'sexual expression', indicating a significant variation in responsiveness of services to different types of need in this patient population. CONCLUSIONS: The MNI seems to be a useful and powerful metric for outcome assessment and monitoring of psychiatric services from a needs assessment approach.