Clinical experience in general hospital psychiatry and literature reviews supported the conjecture that psychopathological disturbances are frequent among medical patients. Wide discrepancies of prevalence data reported by different authors, however, suggested the importance of undertaking screening studies with standardized methods of assessment. Our initial studies in oncological patients confirmed the hypothesis, but also documented obsessive phenomena, assessed with Present State Examination (PSE) criteria, in more than one third of patients diagnosed of depression Consecutive studies in different medical samples have replicated those preliminary findings in the last one, close to one quarter of first day consecutive patients seen in an internal medicine out-patient clinic, and more than three quarters of the ones diagnosed of either anxiety or depression with Research Diagnostic Criteria, had obsessive symptoms as defined by the Clinical Interview Schedule (CIS). Obsessive symptoms, however, have also been observed in individuals considered to be "non-cases" and tend to follow a "continuum" distribution, rather than a categorical one They tend to be more frequent in cases diagnosed as neurotic or reactive, rather than in the endogenous ones and to correlate with neuroticism measured by the EPQ-A. Particularly among out-patients, where the psychopathology seemed to be related to the absence of demonstrable somatic illness and probably to the presence of social distress, the hypothesis could be advanced that obsessive symptoms, among other psychopathological phenomena, are quantitative reactions to environmental situations in predisposed individuals. Nevertheless, in samples such as the endocrine in-patients, correlations have also been demonstrated between obsessive or other psychopathological symptoms and biological deviations such as hormonal levels.(ABSTRACT TRUNCATED AT 250 WORDS)