73-year-old woman complaining of bone pain and weight loss was suspected to have a malignant disease, and extensive laboratory investigations were carried out. She was diagnosed with multiple myeloma; however, because of the finding of extremely high serum levels of CA125 and CA15.3 and focal liver lesions, a concomitant solid tumor was suspected, which was then excluded with the appropriate tests, including an ultrasound-guided liver biopsy. While being diagnosed, the patient developed a rapidly evolving plasma cell leukemia with a simultaneous increase in CA125 and CA15.3. After treatment with cyclophosphamide and dexamethasone, the peripheral blood plasma cells disappeared and there was a dramatic decrease in the CA125 and CA15.3 tumor markers. High levels of the latter can be observed in patients with aggressive plasma cell dyscrasias, an observation that is crucial in order to avoid unnecessary tests that can result in treatment delay.