OBJECTIVES: This study determined the incidence rate, cumulative incidence rate (CIR) and cluster of diagnosis in the patients under PMV (prolonged mechanical ventilation). METHODS: The reimbursement data of National Health Insurance of Taiwan was transformed into a research database by the National Health Research Institutes, in which there were 8,906,406 people who ever used mechanical ventilation at least once during the period of 1997-2007. A random sample of the above population with 3.4:1 ratio provided a database of 2,619,534 subjects under mechanical ventilation for more than 21 days. The incidence rates, cumulative incidence rates were calculated and their characteristics were explored by data mining and cluster analysis. RESULTS: A total of 50,481 new PMV patients were found during the study period with a mean age of 72 years. The age specific incidence rates showed an increased trend as the age grew old. The highest annual incidence rate of PMV in people who were older than 85 years increased from 1,182 to 2,584 per 100,000 between 1998 and 2004, and then decreased 2,046 per 100,000 in 2007. The CIR (17-85 years) increased from 0.103 to 0.183 between 1998 and 2004, and then decreased 0.145 in 2007. The results of data mining showed that about three-fourths of PMV patients developed from generalized infection (septicemia), infections of respiratory system or urinary tract, or cardiovascular diseases affecting lung. The latent class model yielded 3 clusters in patients older than 85 years: septicemia, symptoms involving cardiovascular system and chronic bronchitis. CONCLUSIONS: In addition to chronic bronchitis, septicemia, and cardiovascular diseases complicated with respiratory failure appeared to be the major causes for PMV with a CIR17-85 of 1/7 to 1/10 in Taiwan and deserve future studies of prevention and treatment.