The general aims of this thesis were to evaluate the usefulness of an oral assess¬ment guide and to determine the oral health status among frail patients in hospital settings. A further aim was to analyze possible factors that may be associated with oral health status, with special focus on the relationship between oral health and nutritional status among elderly rehabilitation patients. The Oral Assessment Guide (OAG) was used in patients with haematological malignancies undergoing chemotherapy. In further studies a Revised Oral Assessment Guide (ROAG) was used among geriatric rehabilitation patients. The inter-rater reliability of the OAG between registered nurses at the ward and a dental hygienist (DH) was good, as well as between a registered nurse and a DH using ROAG. Problems in the oral cavity were detected in all patients with haematological malignancies undergoing chemotherapy. Problems related to mucous membranes, teeth and/or dentures were most frequent. Oral health problems were also a frequent finding among the geriatric rehabilitation patients. The frequency of oral health problems was significantly lower at discharge compared to admission. Oral health problems were more often found among patients that stayed for longer periods at the hospital, were more dependent on help with daily activities and suffered for dysphagia, than among the healthier patients. Oral health problems were more common among the patients being at risk of undernourishment, suspected to be or severely undernourished (UN), than among the well-nourished patients. Problems in oral health status were significantly associated with the occurrence of respiratory diseases, living in special accommodation, being UN and being a woman. The highest Odds Ratio (OR) was found in problems with gums in relation to the occurrence of respiratory diseases (OR 8.9; 95 % CI 2.8-27.8). OAG as well as ROAG were found to be useful for assessing oral health status in frail patients at hospital. Oral health problems were a frequent finding among the patients. The use of an oral assessment tool on a routine basis may facilitate the detection of problems which otherwise can be hidden, and can serve as a guide for initiating individualised oral health procedures.