The International Union for Conservation of Nature's Red List of Threatened Species (IUCN Red List) is the world's most comprehensive information source on the global conservation status of species. Countries, governmental agencies, and conservation organizations increasingly rely on IUCN Red List assessments to develop conservation policies and priorities. Additionally, funding agencies use the assessments as evaluation criteria, and research uses meta-analysis of IUCN Red List data to address fundamental and applied conservation science questions. However, the circa 138,000 assessments included in the IUCN Red List to date represent a fraction of the world's biodiversity and the included assessments are biased in regional and organismal cover. It is unknown what proportion of the unlisted taxa are threatened and with an increasingly global conservation agenda, these biases may impact both conservation priorities, funding, and uses of the data for understanding global patterns. Isolated oceanic islands are characterized by high endemicity, but the unique biodiversity of many islands is experiencing high extinction rates, primarily due to habitat disturbance and loss, competition with invasive species, spread of new diseases, and climate change. The archipelago of Hawai'i has one of the highest levels of endemism of any floristic region with 90% of its 1,367 native vascular plant taxa classified as endemic. Of these, 454 (33%) are listed as Threatened or Endangered under the U.S. Endangered Species Act and by the State of Hawai'i. IUCN Red List assessment of all 256 single island endemic vascular plant taxa of Kaua'i reveals 95% of the flora is threatened with extinction and 5% are already extinct using IUCN Red List criteria. This is the highest extinction risk reported for any flora to date. Comparative analysis suggests that the flora of Hawai'i may be one of the world's most threatened calling for urgent conservation measures of remote oceanic island floras. This article is protected by copyright. All rights reserved.