Approximately 9.24% of the Taiwanese population is aged 65 years or older. Among them, osteoporosis is a major problem, along with it associated age-related fractures. We investigated the prevalence of osteoporosis in 1996-2001 by sampling Taiwan's National Health Insurance (NHI) database. Data from 102,763 men (51.27%) and 97,654 women (48.73%) were evaluated. In this cohort, osteoporosis was recorded in each yearly dataset if the codes 733.0 or 733.00-733.09 were found on a search of the administrative or outpatient sub-databanks. A stable estimated prevalence of osteoporosis was calculated according to Taiwan's NHI sampling data from 1999 to 2001. The results showed a trend toward increasing proportions of coded osteoporosis with age, more predominantly in the female population. The averaged prevalence of osteoporosis, between 1999 and 2001, in those aged > or = 50 years was 1.63% for men and 11.35% for women. These estimates were lower than those reported elsewhere for Taiwan and for Japan but more equal to that in the Mexican American sub-population of the United States. In conclusion, the prevalence of osteoporosis is underestimated in the NHI database. Policymakers should be aware of this finding and allocate resources accordingly.