A prospective study was performed to determine acuity levels, waiting times, and reasons why patients who sought care in an emergency department (ED) left without being seen by a physician, and to evaluate whether timed telephone follow-up improved their health outcomes. A comparison and follow-up survey was conducted on all patients who registered for care in the ED at the Naval Medical Center, San Diego, CA, and left without being seen (n = 32) and a 20% systematic sampling of patients who waited until they were seen (n = 170) during a one-week period in Spring, 1993. Baseline demographics and health statuses on reporting to the ED were gathered on all patients waiting to be seen that agreed to participate in the study (n = 533). Demographics for those seen versus those who left without being seen (LWBS) were similar (mean age 23.9 v 23.2, not significant [NS]; percent male 50.5% v 50.0%, NS). Seen versus LWBS had a similar proportion of patients triaged urgent (15% v 6%, NS), and nonurgent (85% v 69%, NS). Twenty-five percent of LWBS left before triage. Acuity levels were compared for patients seen versus LWBS for level I (1.5% v 3%, NS), level II (75% v 78%, NS), and level III (23.5% v 19%, NS). For the 25% that left before triage, level I was 9%, level II was 64%, and level III was 27%. Sixty-nine percent of patients who LWBS were seen for evaluation within 48 hours, 9% in an ED and 60% in a clinic.(ABSTRACT TRUNCATED AT 250 WORDS)