In this cross-sectional study, the illness representations of patients suffering from Chronic Fatigue Syndrome (n=49) and Addison's Disease (n=52) and those of their spouses were compared. Couples generally held similar views with regard to the dimensions of illness identity and cause but disagreed on rime-line, control/cure, and consequences of the illness. The effects of such dissimilarity in a couple's illness representations on the patient's coping behaviour and adaptation were then examined. Dissimilarity was found to have a weak impact on coping and a rather strong impact on adaptive outcome. In general, minimization of the disorder's seriousness by the spouse was found to have a negative impact on adaptive outcome, although this effect differed with the dimension of illness representation and the type of illness. These results demonstrate the importance of the role of significant others and their beliefs for patient's coping and adaptation with chronic illness.