A 23-year-old man diagnosed as having multiple myeloma was treated with melphalan and prednisone monthly. After six cycles, an autologous peripheral blood stem cell transplantation (ABSCT) was planned. Peripheral blood mononuclear cells were collected after G-CSF mobilization (5 micrograms/kg/day for 5 days). Ten days after the last dose of G-CSF the patient showed a striking progression of multiple myeloma. A 57% infiltration of plasma cells in bone marrow and recurrence of laboratory abnormalities were evident. The patient's clinical course strongly suggests that myeloma progression was triggered by G-CSF and supports the concept of G-CSF mediated autocrine stimulation of myeloma growth.