High-dose cyclophosphamide (HD-CY; 7 g/m2) was administered to patients suffering from high risk multiple myeloma (MM). The safety of this procedure, the recirculation and collection of peripheral blood stem cells (PBSC) and the effect of rhGM-CSF and HD-CY were studied. Group I patients (n = 21) were treated with HD-CY alone. Group II patients (n = 10) received 5 micrograms/kg/day rhGM-CSF iv after HD-CY. Neutropenia was shorter in group II (p = 0.01). In group II, the number of circulating colony forming units (CFU-GM) after 14 days was correlated with the number of circulating CFU-GM after 7 days (r = 0.85, p < 0.0001) and with the number of CD34+ cells (r = 0.839, p = 0.01). The total number of mononuclear cells (MNC) and CFU-GM collected per patient was two and seven-fold higher, respectively, in group II (p = 0.01 and p = 0.03). Recovered MNC and CFU-GM were 1.7 and 7-fold higher, respectively, in group II (p = 0.01 and p = 0.004). Our data show that HD-CY is an efficient means of collecting functional PBSC in MM. We suggest that rhGM-CSF is able to further enhance this yield in MM.