To investigate nutritional status and its relationship to cognition, well-being, functional ability and energy intake in frail elderly service flat residents. Cross-sectional and prospective study. Two municipal service flat complexes. A total of 80 residents (median age 85.5 (79-90) y) with regular home care assistance participated. A subgroup of 35 residents took part in a re-examination 1 y later. Mini Nutritional Assessment (MNA), Short Portable Mental Status Questionnaire, Barthel Index and Health Index were used for the evaluation of nutritional, cognitive and ADL function and well-being, respectively. In all, 30% of the frail and chronically ill service flat residents were assessed as malnourished and 59% were at risk of malnutrition. The malnourished residents had worse cognitive conditions (P<0.001) and well-being (P<0.05), lower functional ability (P<0.01) and they had a greater need for daily assistance (P<0.05) than the other residents. The median night fast period was 14.0 (12.5-15.0) h. Five subjects classified as malnourished at baseline had lost a median of -9.6 kg (range -11.0 to +7.3 kg) (P<0.05) in body weight at the 1-y follow-up, which contrasted significantly from the weight stability in residents classified as at risk for malnutrition or well-nourished. Out of 10 residents, nine were assessed to have impending nutritional problems that related to impaired well-being, cognition, and functional ability. Malnourished residents had a significant weight loss over one year. Studies are needed to determine whether weight loss and nutrition-related dysfunction in service flat residents are preventable or treatable.