OBJECTIVE The objective of this study was to conduct a randomised phase II study in second-line metastatic colorectal cancer with the purpose of confirming preliminary clinical data indicating that the formulation of irinotecan with the drug carrier, hyaluronan (HA) reduced toxicity of the drug. METHODS Irinotecan-naïve patients were randomized to receive either irinotecan (350 mg/m(2)) or HA-Irinotecan (HA 1,000 mg/m(2) and irinotecan at 350 mg/m(2)) every 3 weeks for a maximum of eight cycles. RESULTS Seventy-six patients (41 HA-Irinotecan and 35 irinotecan-alone) were enrolled. There was no significant difference in any individual, or overall, grade 3 or 4 toxicity. There was a trend for increased diarrhea in the HA-Irinotecan-treated patients (20 versus 9%; P = 21), potentially explained by a disproportionate number of baseline toxicity-associated risk factors in this treatment group. The median number of cycles completed was six for HA-Irinotecan patients and two for irinotecan-alone patients (P = 0.005). When compared to the control arm, HA-Irinotecan patients had a significantly longer median progression-free survival of 5.2 versus 2.4 months (P = 0.017) and time to treatment failure (4 vs. 1.8 months; P = 0.007). Median overall survival was 10.1 months for HA-Irinotecan compared to 8.0 months for irinotecan patients (P = 0.196). CONCLUSIONS Further studies are required to define the safety of the formulation of irinotecan with HA. While this study was not adequately powered to demonstrate survival differences, these phase II data indicated HA-Irinotecan to be a promising therapy demonstrating improved efficacy compared to irinotecan-alone.