The purpose of this paper is to describe the development and to test the reliability of a new method called INTERMED, for health service needs assessment. The INTERMED integrates the biopsychosocial aspects of disease and the relationship between patient and health care system in a comprehensive scheme and reflects an operationalized conceptual approach to case mix or case complexity. The method is developed to enhance interdisciplinary communication between (para-) medical specialists and to provide a method to describe case complexity for clinical, scientific, and educational purposes. First, a feasibility study (N = 21 patients) was conducted which included double scoring and discussion of the results. This led to a version of the instrument on which two interrater reliability studies were performed. In study 1, the INTERMED was double scored for 14 patients admitted to an internal ward by a psychiatrist and an internist on the basis of a joint interview conducted by both. In study 2, on the basis of medical charts, two clinicians separately double scored the INTERMED in 16 patients referred to the outpatient psychiatric consultation service. Averaged over both studies, in 94.2% of all ratings there was no important difference between the raters (more than 1 point difference). As a research interview, it takes about 20 minutes; as part of the whole process of history taking it takes about 15 minutes. In both studies, improvements were suggested by the results. Analyses of study 1 revealed that on most items there was considerable agreement; some items were improved. Also, the reference point for the prognoses was changed so that it reflected both short- and long-term prognoses. Analyses of study 2 showed that in this setting, less agreement between the raters was obtained due to the fact that the raters were less experienced and the scoring procedure was more susceptible to differences. Some improvements--mainly of the anchor points--were specified which may further enhance interrater reliability. The INTERMED proves to be a reliable method for classifying patients' care needs, especially when used by experienced raters scoring by patient interview. It can be a useful tool in assessing patients' care needs, as well as the level of needed adjustment between general and mental health service delivery. The INTERMED is easily applicable in the clinical setting at low time-costs.