INTRODUCTION: Sudden fever of unknown origin is quite a common emergency and may lead to hospitalization. A rise in body temperature can be caused by infectious diseases and by other types of medical condition. This case report is of a woman who had fever at night for several days and other clinical signs which were likely related to cracked dental mercury amalgam. CASE PRESENTATION: A healthy women developed fever many days after had cracked a mercury dental amalgam filling. Blood tests evidenced increased erythrocyte sedimentation rate, anemia and elevated white cell count; symptoms were headache and palpitations. Blood tests and symptoms normalized within three weeks of removal of the dental amalgam. CONCLUSION: This case highlights the possible link between mercury vapor exposure from cracked dental amalgam and early activation of the immune system leading to fever of unknown origin.