Patients with intact immunoglobulin (Ig) multiple myeloma (MM) usually show parallel fluctuations of their intact Ig and light chain (LC) concentrations. Herein we report 11 patients with relapsed, intact Ig MM with a marked increase in urinary LCs in the absence of a parallel rise in serum intact Ig, known as light chain escape (LCE). A major feature accompanying the presentation of LCE was conversion of plasma cell morphology. An important feature preceding the onset of LCE was the use of biological therapies. These patients were not associated with a highly aggressive course. Median overall survival times after the diagnosis of MM and after LCE were 48 months and 24 months, respectively. These cases represent a documented series of a rare but clinically important mode. More attention should be directed toward patients with intact Ig MM when the clinical course deteriorates despite a reduction of intact monoclonal immunoglobins.