With shrinking military budgets and the inevitable drawdown after the Cold War, the potential erosion of aircrew readiness/proficiency has been a source of concern within the U.S. Department of Defense. Unfortunately, it has been difficult to quantify proficiency using traditional performance measures. However, an analytic tool, the Human Factors Analysis and Classification System (HFACS) was recently developed that has enabled those within the Department of Defense and civilian aviation communities to examine pilot proficiency using accident data. An analysis of U.S. Navy/Marine Corps Class A aviation accidents occurring between fiscal years 1991 and 1998 was conducted using HFACS to determine what, if any, trends were evident that would indicate an erosion of pilot proficiency. Notably, a steady increase in the percentage of accidents associated with skill-based errors was observed beginning in 1991. To the degree that skill-based errors reflect a lack of proficiency, they provide an objective indicator of pilot ability. Several explanations have been suggested for this apparent trend including a reduction in flight hours, changes in aircrew training, and/or pilot retention. By employing HFACS, safety professionals can gain a better understanding of previously nebulous topics such as pilot proficiency.