The efficacy of pegfilgrastim+/-chemotherapy for mobilizing stem cells in patients with solid tumours was assessed. In cycle 0, a 14-day prechemotherapy cycle, patients (N=61) were randomized open-label to single doses of pegfilgrastim (6, 12 or 18 mg) on day 1, or daily filgrastim (10 microg/kg) for < or =7 days. Mean peak peripheral CD34+ cell counts increased with pegfilgrastim dose, but were significantly higher than filgrastim only at the 18 mg dose (10.17 vs 4.96 x 10(4)/ml; P=0.014). In the clinically relevant period of days 3-7, both 12 and 18 mg pegfilgrastim doses produced significantly higher peak CD34+ counts (8.18 and 9.96 vs 4.51 x 10(4)/ml for filgrastim; P=0.034 and 0.006). In cycle 1, patients received carboplatin/paclitaxel on day 1, followed from day 2 by pegfilgrastim 6-18 mg or daily filgrastim (5 microg/kg/day for < or =14 days) as per randomization in cycle 0. There were no significant differences in mean peak CD34+ count between pegfilgrastim and filgrastim, but there was an advantage for pegfilgrastim 18 mg in the relevant period of days 7-12 (3.14 vs 1.19 x 10(4)/ml; P=0.043). A single pegfilgrastim dose (> or =6 mg) could be substituted for daily filgrastim in cytokine-only peripheral CD34+ cell mobilization.