Nurses play a crucial role in providing high-quality care in intensive care units (ICU). Previous studies have shown an association between nurse staffing levels and outcome of critically ill patients. Increasing nurse staffing levels in ICU has been recommended to improve the outcome of critically ill patients. However, nursing staff shortages associated with decreased budgets may prevent adequate nurse-to-patient ratios although there lies increasing needs for critical care. Several studies have suggested that higher nurse staffing level is associated with favorable patient outcome, including mortality, length of stay, and infections, but some of studies did not find an association between nurse staffing and patient outcome. Although there are some controversies in the associations between nurse staffing levels and patient outcome, it is difficult to apply such effect as compared with other developed countries in North American and Europe as the nurse-to-patient ratio in Korea's ICU is relatively low. By studying the nurse staffing effects for patient outcome from the Korea ICU, it is found that higher nurse staffing level is associated with improved patient mortality. This finding may suggest that a shortage of nursing staff is currently a serious issue for caring of critically ill patients in Korea.