A study of the causes of uprooting and stem failure in old-growth giant sequoia (Sequoia gigantea) indicted many factors, depending upon the type of failure (by root, stem or earth). Advanced decay and fire scars were the most frequently associated with failure. In 21 of 33 study trees, one-third or more of the roots were judged too decayed to provide support. Twenty-seven study trees possessed basal fire scars, and 26 fell toward the scarred side. Nine Basidiomycetes, including Fomes annosus, Poria albipellucida, Poria incrassata, and Armillaria mellea, were associated with decayed wood. Carpenter ants were found in or adjacent to the failure zone of nearly half of the study trees. Physical disturbances (e.g., roads, trails, streams) were associated with 22 tree failures, but their role initiating failure requires further investigation.