[AL amyloidosis presenting with fluminant multiorgan failure accompanied by rapid progression from MGUS to multiple myeloma].

Monoclonal gammopathy of undermined significance (MGUS) is usually asymptomatic, and untreated follow-up is the standard treatment. However, MGUS progresses to multiple myeloma or related malignancy at a frequency of 1.5% per year. It is sometimes difficult to diagnose the progression of the disease via usual examinations. We herein report a case wherein rapid renal dysfunction led to a diagnosis of disease progression to multiple myeloma in a patient with MGUS that was asymptomatic for a long time. A 66-year-old woman developed rapid renal dysfunction requiring continuous hemodiafiltration 8 years after the diagnosis of IgA-κ type MGUS. A complete examination led to the diagnosis of IgA-κ type multiple myeloma. Chemotherapy was not effective, and she died due to sepsis on the 19th day of admission. A pathological autopsy revealed systemic amyloidosis and multiple abscesses positive for Staphylococcus aureus. An abnormal free light chain κ/λ ratio and M protein other than IgG are reportedly risk factors of disease progression of MGUS. In cases with these risk factors, it is important to always keep in mind the possibility of disease progression and to monitor the patient carefully for an early diagnosis.