To compare the effectiveness of diverse rehabilitation programmes, comparable data about their effects on maintaining or improving the residual function of the rehabilitation patients should be gathered. Current rehabilitation theories and assessment procedures for functioning are not consistent enough for valid comparisons. The rehabilitation theory should be developed to produce coherence and generalizability to the rehabilitation process. The biopsychosocial disease consequence (BPSDC) model for functioning is presented for this purpose. The model describes the rehabilitation process of patients with chronic pain as a three-axial (biopsychosocial) and three-dimensional (disease consequences) assessment and intervention grid for functioning. It emphasizes the strict mutual relationship between the assessment procedures and intervention plans. Application of the BPSDC model in the Finnish AKSELI project studying the effects of two different programmes on patients with chronic low-back pain is described. Although the AKSELI studies indicated that in addition to the assessment procedures other factors also contributed to valid evaluation of outcomes, and it is hoped that the BPSDC model will encourage researchers to look for definitions of functioning, to assess functioning according to theoretical assumptions about the sub-areas of functioning, and to provide comparable outcome data for the evaluation of various programmes.