BACKGROUND: Empathic concern (EC) is an important interpersonal resilience factor that represents positive adaptation, such as "relating to others" (a factor of posttraumatic growth [PTG]) after disaster. However, controversy exists regarding whether the changes in EC (e.g., the intra-personal change between the acute phase and the disillusionment phase) positively or negatively affect mental health after a disaster. We hypothesized that increased EC may increase chronic fatigue due to over-adjustment (hypothesis 1). We also hypothesized that increasing the changes in "relating to others" could decrease the changes in chronic fatigue (hypothesis 2). METHODS: Forty-nine young, healthy volunteers (M/F: 36/13; age at 3 months after the disaster [3 months]: mean ± SD: 21.1 ± 1.7 years) underwent assessments of EC using the Japanese version of the Interpersonal Reactivity Index, chronic fatigue using the Japanese version of the Checklist Individual Strength (CIS-J) questionnaire, and "relating to others" using the Japanese version of the PTG inventory during the acute phase (3 months) and the disillusionment phase (1 year after the disaster). Pearson product moment correlations at 3 months and 1 year were determined for all scores related to EC. The changes (delta = degree of change from 3 months to 1 year) or scores at 1 year were entered into linear structural equation systems to test the hypotheses. RESULTS: The delta of EC positively affected the delta of the CIS-J, and the delta of relating to others negatively affected the delta of the CIS-J. Both the EC and relating to others scores were negatively associated with the CIS-J score at 1 year. These results were in accordance with hypothesis 1 and 2. CONCLUSIONS: We demonstrated the opposite effects of 2 types of ECs, i.e., stability (inherent disposition) and flexibility (degree of change), on the degree of chronic fatigue. Increasing EC with increasing chronic fatigue, but not the change in relating to others, may be a red flag for individuals during the disillusionment phase.