A 53-year-old woman presented to the orthopedic department with severe diffuse muscular and bone pain. An x-ray of her right upper extremity revealed a lytic destructive lesion in the right humerus. Computed tomography scans showed multiple lytic lesions in the spine and pelvis, and a biopsy confirmed the presence of 70% plasma cells, which were κ light chain restricted. The patient was referred to a hematologist. Although no monoclonal protein was detected in the serum by protein electrophoresis or by immunofixation electrophoresis (IFE),1 the κ free light chain (FLC) was increased at 47.2 mg/L (reference interval, 3.3–19.4 mg/L), with a κ/λ FLC ratio of 23 (reference interval, 0.26–1.65). Serum concentrations of β2-microglobulin and albumin were 248 nmol/L (reference interval, 59.5–153 nmol/L) and 37 g/L (reference interval, 34–47 g/L), respectively. The urine protein concentration was not increased, but protein electrophoresis revealed a small M (monoclonal) spike in the γ region (32 mg/24 h). In addition, IFE identified a monoclonal κ light chain (Bence Jones protein). On the basis of these findings, the patient was informed that she had stage I (International Staging System) oligosecretory/nonsecretory multiple myeloma (MM). The patient underwent surgical repair of her right humerus and was evaluated 1 month after her surgery. At that point, a second serum FLC measurement showed a κ FLC concentration of 68.2 mg/L. The patient’s hematologist recommended close observation in lieu of initiating therapy because of her lack of symptoms. The patient’s serum κ FLC concentration was monitored monthly and remained <50 mg/L for the next 3 months. Five months after diagnosis, the patient began to complain of mild fatigue, shortness of breath, and palpitations. A 10-fold increase in the urinary M protein to 333 mg/24 h was noted, along with a decrease in the blood hemoglobin concentration to 79 g/L …