In order to investigate the effects of erythropoietin (EPO) plus granulocyte colony-stimulating factor (G-CSF) administration after peripheral blood progenitor cell transplantation (PBPCT) we performed a phase I/II study in patients with high-risk cancer. 15 consecutive patients were treated wit recombinant human G-CSF (rhG-CSF) at the dose of 5 micrograms/kg subcutaneously (s.c.) every 24 h until day + 12 and with recombinant human EPO (rhEPO) at the dose of 150 IU/kg s.c. every 48 h until day + 11 following PBPCT. Their haemopoietic recovery was compared to that obtained in eight historic and control patients who did not receive any cytokines after PBPCT. No side-effects were observed during EPO plus G-CSF treatment and the treatment was not discontinued in any of the patients before completion of the treatment plan. The administration of EPO plus G-CSF after PBPCT produced a significant increase in the rate of white blood cell (WBC) (P = 0.0005), polymorphonuclear leucocyte (PMN) (P = 0.0005) and platelet (PLT) (P = 0.0105) recovery compared to the control group. The acceleration in haemopoietic recovery observed in the EPO plus G-CSF-treated patients produced a significant reduction of the days with WBC < 1 x 10(9)/l (P = 0.0009), PMN < 0.2 x 10(9)/l (P = 0.0030) and PMN < 0.5 x 10(9)/l (P = 0.0006). EPO plus G-CSF-treated patients required a significantly lower number of single donor PLT transfusions (P = 0.0142) and did not experience neutropenic fever, but historic control patients experienced fever > 38 degrees C for a median period of 4 d (0-12) with a medial period of parenteral antibiotic administration of 7.5 d (0-17). The length of the hospital stay was significantly shorter in the study group than in the historic control group (P = 0.0264). In conclusion, we can confirm that EPO plus G-CSF treatment is feasible and potentiates the haemopoietic recovery after PBPCT, thus simplifying the clinical management of cancer patients who undergo high-dose chemotherapy.