Clinical experience indicates that physical diseases are probably underdiagnosed in patients suffering from dementia. We investigated the prevalence of physical diseases in patients with different types of dementia by means of a retrospective patient record survey including 236 inpatients and outpatients referred for dementia evaluation to the Dementia Investigation Unit, University Hospital in Linköping during 1994. Forty-four patients had dementia of the Alzheimer type, 78 had vascular dementia, 28 had dementia due to multiple etiologies, 42 were not demented, and 44 patients could not be classified by the DSM IV criteria. The physical diseases were registered as separate diagnoses comprising all newly-diagnosed physical diseases and previously known diseases that had exacerbated and contributed to the medical contact. Sixty-four percent of the patients had previously unknown physical diseases and/or exacerbation of previously known diseases. The most common physical conditions were cobalamin deficiency and infectious diseases, which occurred in 27% and 24% of the patients, respectively. There was no difference in the number or kinds of diagnoses between the diagnostic groups. Associated physical diseases were underdiagnosed in patients referred for dementia evaluation. We suggest that thorough medical investigation and adequate treatment are of importance in the management of dementia.