The purpose of this study was to describe nationally representative characteristics and temporal trends in "left before being seen" (LBBS) visits in US emergency departments (EDs). The ED portion of the federal National Hospital Ambulatory Medical Care Survey, 1995-2002, was analyzed. Of the 810.6 million ED visits during the 8-year study period, an estimated 11.4 million (1.41%, 95% confidence interval [CI] 1.30-1.52) had an LBBS disposition. The number and proportion of LBBS visits have increased over time, from 1.1 million visits in 1995 (1.15%, 95% CI 0.95-1.35) to 2.1 million visits in 2002 (1.92%, 95% CI 1.67-2.17). LBBS patients were more likely to be younger, non-White, Hispanic, urban, and uninsured compared to non-LBBS patients. The number and proportion of LBBS visits have increased over time. LBBS visits disproportionately affect vulnerable populations. These findings suggest that recent strains on the US ED system are adversely affecting healthcare quality and access.