Objectives. To examine Lazarus and Folkman's (1984) stress theory regarding the effects of the stress mediators information and perceived social support on anxiety (as the stress response) regarding gastrointestinal (GI) endoscopy (as the stressor) in male and female patients of various age groups. Design. Non-experimental design. Methods. Structured interviews were conducted with 113 hospital out-patients about to undergo GI endoscopy. Participants indicated their perceptions of how much support and how much clear and useful information they had received from both their general practitioner (GP) and a patient information leaflet developed in collaboration with health psychologists as well as their perceptions of how much social support they had obtained from other patients, family and friends. Anxiety was measured with a population-specific trait and state adaptation of the Hospital anxiety and depression scale (HADS-A). Results. Psychometric exploration of the HADS-A revealed a single general anxiety factor. The reliability of this factor was high, with Cronbach's α = 0.91. The majority of the sample experienced high anxiety levels. Gender, but not age, differences emerged, showing females to be more anxious than males, F(1, 84) = 5.68, p < .05. A regression model built on stress theory was tested, with anxiety as the dependent variable and 11 predictor variables. The model was significant with R2 = 0.452, F(11, 47) = 3.522 and p = 0.001. Conclusions. The clarity, but not the amount, of information and social support from important others, but not GPs, were both mediating the stress experience of the patients by reducing their perceived anxiety.