The increasing mechanical reliability of Naval Aviation (NA) aircraft has made human error (HE) the leading cause of Class A and B flight mishaps. To increase understanding of the underlying causes of HE, the Naval Safety Center in Norfolk, VA, developed the Human Factors Analysis and Classification System (HFACS). The HFACS taxonomy consists of 17 types of basic HE. This HFACS taxonomy has been used as a data analysis tool to classify 141 Class A and B Naval Tacitcal Aircraft (TACAIR) flight mishaps (FM) from fiscal year (FY) 90 and FY97. The study shows an important relationship between Adverse Mental State and 12 of the 17 HE types in the HFACS taxonomy. Significantly, when one of these 12 HE types is cited in and FM, greater than 70 percent of the time, Adverse Mental State is also cited as a co-casual factor in the mishap. Two other HE types have this 70 percent co-causal factor relationship with 3 of the 17 HE types. Adverse Mental State has an important relationship to the majority of the HE types in the HFACS taxonomy, compared with other HE types. For this reason, Adverse Mental State merits further investigation and should be considered in the development of HE prevention programs for Naval Aviators.