The emergency room (ER) is the gatekeeper of the hospital. It receives clients seeking help from the health care system, then refers them for necessary further care or, discharges them back into the community. A 1-year randomized control trial was conducted in an acute general hospital in Hong Kong to see if post-ER nurse follow-up helped to change health outcome and health care utilization. The intervention group received two follow-up calls from an experienced ER nurse, within 1-2 days and 3-5 days after ER discharge. The calls' content and the management options decisions were protocol driven. A total of 795 patients (intervention group=395, control group=400) completed the study. Bivariate analysis shows two significantly different variables between the intervention and control groups, improvement of the condition and ER revisit within 30 days. When other factors are controlled in a multivariate analysis using logistic regression, the effect of the intervention on re-visits to ER within 30 days is reversed. Gender, times of attending general practitioner after ER visits, and not considering other doctors are risks factors related to a higher chance of re-visiting ER within 30 days. Those who have an improved health condition and higher number of times attending general outpatient after ER visits are associated with a lower 30-day ER revisit rate. Nurse telephone follow-ups might have sensitized subjects to health care needs. Some subjects tended to use the ER as a primary care setting and some were doctor-shoppers. A more structured ER transitional model that incoporates nurse telephone follow-up and better interfacing between private and public health care sector, primary and tertiary care might help to decrease inappropriate ER use.