We treated 103 multiple myeloma (MM) patients with 7 g/m2 cyclophosphamide (Cy) followed by 300 micrograms G-CSF/d to harvest peripheral blood progenitor cells (PBPC). PBPC autografts containing > 2.0 x 10(6) CD34+ cells per kg body weight were obtained at the first attempt from 90/100 evaluable patients. The most significant factor predicting impairment of PBPC collection was the duration of previous melphalan treatment (P < 0.0001). In multivariate discriminate analysis, treatment with melphalan during the most recent chemotherapy cycles prior to mobilization (P = 0.0727) and previous radiotherapy (P = 0.0628) had a marginally significant negative influence on the efficacy of PBPC collection. We found no reduced functional capacity of CD34+ cells to restore haemopoiesis after myeloablative treatment related to the duration of melphalan exposure. At the time of best response to conventional treatment, a median paraprotein reduction of 21% was achieved following high-dose cyclophosphamide (HD-Cy). Two heavily pretreated patients died and one patient developed pulmonary toxicity W.H.O. grade IV following HD-Cy. Potential transplant candidates should undergo mobilization and harvesting of PBPC before melphalan-containing treatment. Combinations of haemopoietic growth factors and their dose-modifications should be investigated to improve PBPC collection, to allow a dosage reduction of the mobilization chemotherapy.