The self-serving attributional bias was studied in matched groups of patients with persecutory delusions, patients with major affective disorder, and normal controls, (N=14 in each group). On a preprogrammed computer task, subjects mainly won points in one condition and mainly lost points in the other. Subjects were asked to estimate the degree of control they thought they had over winning or losing in the two conditions. In comparison with the normal subjects and the psychiatric controls, the deluded subjects showed a greater selfserving bias, as evidenced by their perceived greater control over outcomes in the win condition. These findings are interpreted as consistent with the hypothesis that persecutory delusions function as a defense against low self-esteem.