The purpose of this study was to evaluate the HIV prevalence rate in adults of Taiwan up to the end of 2003. As well as estimating the prevalence rate, the possible coverage rate of the reported cases, and the numbers of unreported cases were evaluated. These results will not only provide an important reference for implementation of future prevention policy of HIV/AIDS in Taiwan, but will also form a basis for comparison with international HIV epidemics. The tool was the 2003 version provided by UNAIDS/WHO, for making estimates of HIV/AIDS in countries with low-level or concentrated epidemics. The workbook method was used as the core computation platform to calculate the number of HIV/AIDS infections in Taiwan. Our result shows that an estimated 9,547 persons (range 3,934-17,612) amongst adults aged 15-49 years in Taiwan were living with HIV at the end of 2003. The estimated HIV prevalence rate of adults was 0.07% (range 0.03%-0.14%). Compared with the 3,912 persons with HIV/AIDS reported by Taiwan Center for Disease Control in 2003, a possible 5,635 people remained unreported. Moreover, the ratio of reported case numbers to unreported ones was 1:1.4. In other words, the potential number of HIV cases could be 2.4 times (range: 1-4.5) the number of reported ones. When compared with similar Asian countries, the HIV prevalence of Taiwan is slightly lower than that of Singapore, close to that of Hong Kong and higher than that of Korea and Japan. The coverage rate of reported cases is 42% in Taiwan, which is similar to that of Japan. The data indicates that, although Taiwan still remains at the stage of ”low prevalence epidemic”, there is a large discrepancy between reported and unreported cases. It is therefore strongly suggested that more is invested in developing effective surveillance techniques in order to identify all the unreported cases.