The aim of the study was to identify the phenomenological characteristics of those delusions which are associated with action. The sample consisted of 79 patients admitted to a general psychiatric ward, each of whom described at least one delusional belief. The variables studied included the phenomenology of the delusions, and behaviour. Two behavioural ratings were used, one derived from the subjects' own description of their behaviour and the other from information provided by informants. There was no association between delusional phenomenology and acting on a delusion when the subjects' behaviour was described by informants. When action was described by the subjects themselves, acting was associated with: being aware of evidence which supported the belief and with having actively sought out such evidence; a tendency to reduce the conviction with which a belief was held when that belief was challenged; and with feeling sad, frightened or anxious as a consequence of the delusion.