OBJECTIVE: To assess the dental treatment backlog and associated costs among institutionalized elderly people using a novel composite index, called the oral health index. BACKGROUND: Despite numerous reports about oral health of old individuals, there is still lack of a systematic and practical method to estimate their treatment need covering all relevant aspects of oral health. In addition, little has been published on associated treatment costs and prediction of such costs. MATERIALS AND METHODS: An observational study was performed on 143 dentate institutionalized elderly people, whereby several clinical parameters were registered. The collected data were included in the oral health index representing the need for oral health care. This covered the number of caries lesions, number of residual roots, periodontal health condition, plaque score and denture condition with a final score of 0-9. To investigate the validity of the index, the treatment costs were estimated using the measured clinical parameters and later compared with the actual expenditure of the patients for the following 2 years. RESULTS: The average score of the index was 4.6 (SD 1.4) with 65.1% of the individuals having a score between 3 and 6 and 27.3% having a score of 6 and more, exhibiting medium and high need for oral health care, respectively. Only 30% of the patients underwent all the indicated treatments. The major reason of non-completion of the treatment was patient's refusal. From the fully treated group, 61.5% of the subjects actually spent within the predicted range while 38.5% of them spent more than estimated. The underestimation was related to yearly calculus removals leading to repeated calculation of the same costs and newly emerging dental problems (33% developed new caries and 20% was confronted with tooth fracture within the 2-year period). CONCLUSION: The novel index can be helpful to determine oral treatment needs and associated costs. Further research is needed to extend the clinical applicability of the index.