In a context of a "continuity of care" model in Psychosomatic and Liaison Psychiatry, this study is intended to test, in patients to be discharged from Medicine wards, hypotheses related to a high prevalence of depression and, in particular, its negative outcome at six-months follow-up in Primary Care (PC), specially among the elderly. Methods:Sample. Consecutive patients aged 18 years or more, hospitalized in Medicine wards were randomly selected for screening at the time of admission. On the bases of a previous study and the expected sampling errors, sample size was estimated in 700 patients (approximately 60% in geriatric age) for the initial screening, to recruit 75 or "DEPRESSIVE CO-MORBIDITY IN MEDICAL IN-PATIENTS AT THE TIME OF HOSPITAL... 173 more cases of depression and enough number of control, non-cases without psychiatric morbidity.. Instruments. Standardized, Spanish versions of screening/case-finding instruments COMPRI/INTERMED, Mini-Mental, CAGE and drug screening, Hospital Anxiety and Depression Scale (HADS), Standardized Polyvalent Psychiatric Interview (SPPI). Diagnostic criteria ICD-10 research, medical patients version and DSM-IV-TR (psychiatric); and ICD-9-M; ICHPPC, WONCA (medical). Cumulative Illness Rating Scale (CIRS) (severity of physical condition); SF36 and EuroQol (quality of life); Client Service Receipt Interview (CSRI) (costs). Procedure. Part I, hospital study: Two-phase screening (lay interviewers: COMPRI/INTERMED at admission; Mini-Mental, CAGE, HADS at the time of discharge; and standardized clinicians: SPPI). CIRS was used to control severity of physical conditions. Part II, follow-up study in PC (six months): Standardized clinicians, blind to the previous phases (HADS and SPPI to both cases and controls). Outcome study: Euro-Quol, SF36, CSRI and data on morbidity and mortality, were collected. Conclusions. To our knowledge, this is the first study using modern epidemiological methods in medical patients to be discharged with co-morbid depression and followed in PC with a "continuity of care" strategy. The final analysis of data should support the design of an evidence-based, intervention study on co-morbid depression.