Background: For optimizing the validity of diagnoses of mental disorders, several sources of information should be used to assess psychopathological symptoms. Among these are relatives of patients with mental illness. The very low number of empirical studies examining the assessment of psychopathology by relatives of adult, nondemented mentally ill patients stands in significant contrast to the clinical importance of this source of information, however. Sampling and Methods: Using the Positive and Negative Syndrome Scale (PANSS), researchers asked 163 relatives of patients with the main clinical ICD-10 diagnosis of schizophrenic, recurrent depressive or bipolar disorders to rate the current symptoms of the patients at the time of outpatient community-oriented treatment. Results: On average, severity of symptoms was rated as absent or minimal, although anxiety, depression and passive/apathetic social as well as emotional withdrawal, motor retardation, poor attention, and disturbance of volition were clearly rated above the PANSS mean total score for all patients. A six-factor structure identified by factor analysis better illustrates the significant differences in the assessments of the three main diagnostic groups than the three established PANSS scales. With the exception of ‘problematic social behavior’, differences among the diagnostic groups appeared in all factors and were particularly pronounced for ‘delusional beliefs’ and ‘motor impairments’. Conclusions: The results of this study showed that the use of standardized instruments such as PANSS for the assessment of psychopathology by relatives is not only practical, but produces adequately reliable results. The use of PANSS for this purpose, however, requires interviewing of relatives by trained experts able to explain technical terms. Because this study did not sufficiently explore the validity of this approach, further research on this specific issue is urgently needed and should, for example, assess the concordance of ratings between professionals and relatives as well as correlation with suitable external criteria.