Several overlapping features have frequently been described between psychosis and the subjective experience of dreaming from the neurobiological to the phenomenological level, but whether this similarity reflects the cognitive organization of schizophrenic thought or rather that of psychotic mentation independent of diagnostic categories is still unclear. In this study, 40 actively psychotic inpatients were equally divided in two age- and education-matched groups according to their diagnosis (Schizophrenia and Bipolar Disorder). Participants were asked to report their dreams upon awakening and the Thematic Apperception Test (TAT) was administered to elicit waking fantasies; the same procedure was used in a control group of 20 non-psychiatric subjects. Two highly trained judges scored the collected material according to a Dream Bizarreness scale. The same level of cognitive bizarreness was found in TAT and dream reports of schizophrenic and manic subjects but was almost completely absent in the TAT stories of the control group. Two-way analysis of variance for repeated measures assessed the effect of diagnosis and experimental conditions (TAT stories and dream reports) on bizarreness yielding a significant interaction. Cognitive bizarreness seems to be a shared feature of dreaming and psychotic mentation, beyond diagnostic categorizations. Although these findings must be considered preliminary, this experimental measure of the cognitive architecture of thought processes seems to support the view that dreaming could be a useful model for the psychoses.