BACKGROUND: Emergency departments deal with large patient loads on a day-to-day basis. The importance of patient self-assessment in the triage process has not been fully considered when determining the need for hospital admission. OBJECTIVE: To determine the validity of a series of self-administered triage questions in determining the need for hospitalisation in the emergency setting. DESIGN: Prospective cohort study. SETTING: Emergency department in a tertiary-care, municipal hospital in Japan. PARTICIPANTS: 5380 consecutive walk-in patients visiting the emergency department of Okazaki City Hospital were asked to self-evaluate the urgency and severity of their condition and their perceived need for hospital admission. These patients were then assessed by emergency physicians blinded to the results from each patient's self-assessment. MAIN OUTCOME MEASURES: Sensitivity, specificity and likelihood ratios were calculated for each self-assessment by comparing these with findings from assessments made by emergency physicians. RESULTS: Patient-perceived need for hospitalisation had a sensitivity of 0.79 (95% CI 0.76 to 0.82) and a specificity of 0.93 (95% CI 0.92 to 0.93) in determining hospital admission. The positive and negative likelihood ratios for self-assessments were 10.68 (95% CI 9.59 to 11.90) and 0.22 (95% CI 0.19 to 0.26), respectively, in the diagnosis of hospital admission (p<0.01). CONCLUSIONS: The patient self-triage questions concerning condition with five categories (medication only to life threatening) seem to supplement the triage process for hospital admission in emergency departments.