We use the stigma of illness as a model for uncovering which dimensions of stigmas are critical for causing social rejection. Subjects responded to ‘medical case histories’ representing 66 illnesses, rating the illness on a variety of dimensions (e.g. contagious/not contagious, common/rare), and a measure of social rejection. Regression analysis revealed that two dimensions predicted rejection: the severity of the illness, and whether the illness was behaviourally caused (multiple R = .68). In a second experiment, subjects responded to a case history of a fictitious disease that was either mild or severe, and was either behaviourally caused or not. Behavioural causality and severity emerged as reliable, independent sources of rejection. Diseases perceived to be severe or under personal control are most likely to lead to social rejection.