BACKGROUND: Somatoform symptoms are frequent and are associated with mental distress in several settings and in the general population. Many of these interactions are not well understood or not understood at all. OBJECTIVE AND METHODS: Based on bibliographical research, to investigate: a) the history of somatization-related illnesses; b) its many categories according to contemporary psychiatric and non-psychiatric concepts (somatoform disorders, dissociative-conversive disorders, chronic fatigue syndrome, fibromyalgia, irritable bowel syndrome etc.); c) associated factors and etiological hypothesis for these phenomena. Using data available from the University of São Paulo Institute of Psychiatry Epidemiologic Catchment Area Study (Andrade et al., 1994) - a cross-sectional survey of physical and psychiatric morbidity which interviewed a representative sample of São Paulo city boroughs of Vila Madalena and Jardim América - to explore the prevalence and classification of reported physical symptoms in general, and specifically those which are medically unexplained (MU). To appraise, using logistic regression, the following associations: a) somatoform syndromes and demographic variables; b) somatoform syndromes and use of medication and health services; c) MU symptom and any type (AT) of symptom with demographic variables; and d) MU and AT physical symptoms with mental disorders. To probe, through latent class analysis of yes or no answers for 36 MU physical symptoms: grouping of subjects in classes (cluster analysis); and b) grouping of symptoms in dimensions (factor analysis). One-thousand, four-hundred and sixty-four interviews were performed using the Composite International Diagnostic Interview (CIDI), structured diagnostic interview for mental disorders, including somatoform and dissociative disorders; and a form of demographic variables, chronic physical diseases, use of medical and psychological services and medication intake. RESULTS: No association between somatoform syndromes and demographic variables. Association between somatoform syndromes and increased use of services and medication intake. Association between demographic variables and certain MU/AT symptoms. Physical symptoms AT/MU are associated with mental disorders, in general. Some physical symptoms (fainting, dyspnea, chest pain, arm or leg pain, tingling, heavy menses, muscle weakness, lump in throat, polyuria, palpitations and dizziness) are associated with many of the studied mental disorders. Subjects were clustered into two classes, with low and high symptom scoring; the high scoring class was associated with physical diseases, mental disorders, use of health services and medication intake. Symptoms were grouped into one single factor, pointing to the existence of a somatization dimension in the general population.