Background: Hypertension (HTN) is a serious health problem that threatens one fourth of the adult population in some countries. Objectives: This study aimed to assess the prevalence and outcome of undiagnosed hypertensive patients admitted to the emergency department. Materials and Methods: This cross-sectional study was conducted from March 2009 to March 2010 at Imam Hossein Medical and Educational Center, Teheran, Iran. A total of 2070 patients aged 18 years and older were admitted to the emergency department without previous HTN history. Blood pressure was taken and repeated 10 minutes later if initial systolic blood pressure ≥ 140 mmHg or diastolic blood pressure ≥ 90 mmHg. Those who matched the inclusion criteria entered the study for further follow-up. A numerical pain score was also used for pain intensity assessment. Chi-Square and Mann Whitney U tests were performed to compare differences between sex, age and education of the participants. Results: Based on the inclusion criteria, 346 patients entered the study, out of which 168 qualified for further evaluation and follow-up. Forty eight patients (28.6%) were finally diagnosed with high blood pressure. Our study showed that the prevalence of undiagnosed HTN was 4.8%. Significant differences between blood pressure, age, pain score and education level (P < 0.001) were found. This implies that old age, poor education and low pain score are positively associated with hypertension. Conclusions: Blood pressure readings in emergency departments should not be readily attributed to pain or anxiety. Diagnosis must be based on meticulous follow-up and precise examinations.