BACKGROUND: Although delusions are the hallmark of major depression with psychotic features, a scale to measure the intensity of beliefs across multiple delusional domains in this condition has been unavailable. The development and assessment of the Delusional Assessment Scale (DAS) are described. METHODS: Scale items were selected initially based on previous studies of delusional ideation in schizophrenia. A three-point item to assess mood congruence was added. A 15-item scale was assessed in 92 subjects participating in the four-site collaborative study of the pharmacotherapy of major depression with psychotic features. Maximum likelihood method was used to determine scale factors. The internal consistency of these factors was determined. Comparisons between scale scores and ratings from the Brief Psychiatric Rating Scale (BPRS) (Overall and Gorham 1962) were used to assess convergent and discriminant validity. RESULTS: The data were fit by a five-factors model (impact, conviction, disorganization, bizarreness, and extension). Inter-rater reliability of the five factors ranged from .77 for conviction and .74 for impact to .37 for disorganization. Internal consistency for each of the five factors was > or =.72. Scores on specific domains were significantly correlated with the BPRS unusual thought content item and positive symptom subscale scores. CONCLUSIONS: The DAS is a reliable measure of 5 delusional domains.