A 68-year-old patient presented with a two-week history of fatigue, weakness, and sleep disturbances. This followed a curative prostatectomy due to cancer and an upper respiratory tract infection. Initially, obstructive sleep apnea (OSAS) was diagnosed. Several weeks later, new symptoms of dyspnea emerged, suggesting heart failure. Ultimately, AL-amyloidosis with cardiac involvement, associated with smoldering multiple myeloma, was diagnosed. Early consideration of a hematological malignancy, supported by immunofixation electrophoresis and closer monitoring, could have led to a quicker diagnosis and improved prognosis.