This study aims to detect different psychopathological dimensions in first-episode psychoses with different underlying causes. We evaluated 22 subjects with first-episode psychosis, who differed in biological variables (HIV-positive versus HIV-negative) and who were compared by using the Structured Clinical Interview for DSM-III-Reviewer, the 18-item Brief Psychiatric Rating Scale (BPRS), the 17-item Hamilton Depression Rating Scale, the 14-item Hamilton Anxiety Rating Scale and the Mini-Mental State Examination. HIV-positive subjects had higher mean scores on the global BPRS and on the paranoid Positive and Negative Syndrome Scale subscale compared with HIV-negative subjects. Conversely, higher prevalence of affective and anxious symptoms was found in the HIV-negative patients in comparison to HIV-positives. HIV-positives had significantly greater attention/concentration impairment than HIV-negative persons. In conclusion, taking into account psychopathological dimensions may help psychiatrists in clinical decision-making regarding the differential diagnosis of psychotic symptoms. The psychopathological pattern of first-episode psychosis in HIV-positive patients may represent an 'elementary model' of acute psychosis characterized by paranoid delusions in the absence of the usual affective symptoms.