DSM III-R acknowledges that delusional disorder and mood disturbance can coexist. The aim of the study is to analyze mood disturbances occurring within a delusional disorder. An external validator, as the increased familial risk of psychiatric disorder, is also added to clarify the relationship between the two diagnostic areas. We found a high frequency of mood disturbances in our patients (50.7%). We were able to identify a proportion of delusional patients affected by a recurrent form of mood disturbance (35.2%); in about 42% of these patients the onset of the mood disturbance preceded the onset of the delusional disorder by a considerable interval of time. Our hypothesis is that, in these cases, the observed mood disturbance could represent a codiagnosis of true mood disorder. This hypothesis is partly supported by familial data.