Illness beliefs are important predictors of psychological outcome in people with chronic illness and evidence suggests these could also be significant in furthering our understanding of psychological functioning in people with Parkinson’s disease. Illness beliefs are specific, dynamic representations of an illness and cover dimensions such as cause, identity, consequences and controllability. Eighty-one people with Parkinson’s disease completed a series of questionnaires to provide demographic, clinical and psychosocial data, which were then used to assess the relative impact of illness beliefs on their psychological functioning. Psychological functioning was assessed by measuring levels of depression, anxiety, stress, positive affect and emotional well-being. Hierarchical block regression indicated that illness beliefs were important independent predictors across some but not all outcomes and the results emphasised the importance of testing new predictors against more established predictors of outcome such as physical functioning and self-esteem. The illness beliefs most important in psychological outcome in people with PD were causal beliefs (particularly in psychosocial causes) and illness coherence (the level of understanding of the illness). The therapeutic potential of psychosocial variables was discussed given that these can be modified during therapy and this change can positively influence psychological outcome.