Background: Mental disorders may interfere, aggravate or mimic medical conditions. Aim: To study the prevalence of psychiatric disorders among patients hospitalized in a medical ward of a general hospital. Patients and methods: A structured interview for DSM-III, devised for "non patients", was applied to 203 men and 203 women, aged 11 to 90 years old, hospitalized in an internal medicine service of a public hospital. Thirty four psychiatric conditions that can be discriminated with the instrument and others that complied with DSM-III criteria, were investigated. Results: There was a 60% prevalence of mental disorders among men (alcohol dependency in 26%, delirium or dementia in 10.8%, anxiety disorders in 10.4%, major depression in 7.8% and adaptation disorders in 3%). Among women, the prevalence of mental disorders was 65% (major depression in 23.2%, anxiety disorders in 14.3%, adaptation disorders in 8.4%, dementia in 5.6%, delirium in 3% and alcohol dependency in 2.5%). Most conditions were of moderate or mild intensity. Two or more conditions coexisted in 40% of cases. Only in 8% of these subjects, a psychiatric consultation was requested. Conclusions: There is a high frequency of psychiatric disorders among medical patients. These must be adequately diagnosed and treated (Rev Méd Chile 2001; 129: 1279-88)