OBJECTIVE The aim of this study was to investigate the suitability of existing definitions of ambulatory care sensitive conditions (ACSC) in the setting of an emergency department (ED) by assessing ACSC prevalence in patients admitted to hospital after their ED stay. The secondary aim was to identify ACSC suitable for specific application in the ED setting. DESIGN Observational clinical study with secondary health data. SETTING Two EDs of the Charité-Universitätsmedizin Berlin. PARTICIPANTS All medical ED patients of the 'The Charité Emergency Medicine Study' (CHARITEM) study, who were admitted as inpatients during the 1-year study period (n=13 536). OUTCOME MEASURES Prevalence of ACSC. RESULTS Prevalence of ACSC in the study population differed significantly depending on the respective ACSC set used. Prevalence ranged between 19.1% (95% CI 18.4% to 19.8%; n=2586) using the definition by Albrecht et al and 36.6% (95% CI 35.8% to 37.5%; n=4960) using the definition of Naumann et al. (p<0.001). Overall ACSC prevalence (ie, when using all diagnoses used in any of the assessed ACSC-definitions) was 48.1% (95% CI 47.2% to 48.9%; n=6505). Some frequently observed diagnoses such as 'convulsion and epilepsy' (prevalence: 3.4%, 95% CI 3.1% to 3.7%; n=455), 'diseases of the urinary system' (prevalence: 1.4%; 95% CI 1.2% to 1.6%; n=191) or 'atrial fibrillation and flutter' (prevalence: 1.0%, 95% CI 0.8% to 1.2%, n=134) are not included in all of the current ACSC definitions. CONCLUSIONS The results highlight the need for an optimised, ED-specific ACSC definition. Particular ACSC diagnoses (such as 'convulsion and epilepsy' or 'diseases of the urinary system' and others) seem to be of special relevance in an ED population but are not included in all available ACSC definitions. Further research towards the development of a suitable and specific ACSC definition for research in the ED setting seems warranted. TRIAL REGISTRATION German Clinical Trials Register Deutsches Register für Klinische Studien: DRKS-ID: DRKS00000261.