Purpose: This study was to compare and examine the factors influencing burden of primary family caregivers according to the severity of illness of elderly patients admitted in an intensive care unit. Methods: Subjects were the families of elderly patients in intensive care units of K, S and Y hospitals in Seoul. Data were collected from March to October 2007. Subjects were 108 persons over age 65. Data were analyzed by SAS statistics. Results: First, groups 5 and 3 showed higher burden than that of group 4. Second, high correlation was found between stress and burden, stress and anxiety, and burden and anxiety. Third, factors influencing family burden were found to be stress for group 5, stress, anxiety, and monthly income for group 4, and stress and patient age for group 3. Conclusion: Specific nursing interventions to decrease the stress of primary family caregivers of serious ill elderly patients in an intensive care unit are needed. Additionally, more effective and systematic activation of a long-term medical insurance system for seriously ill seniors is considered necessary to mediate the burden of primary family caregivers.