The purpose of the study was to evaluate the influence of baseline haemoglobin level in predicting response to 5-fluorouracil (5FU)-based first-line chemotherapy in advanced colorectal cancer patients. Data from 631 patients were collected from three different institutions. Globally, overall response rate was 35.8% (226 out of 631). Factors influencing response rate were 5FU dose intensity (high: 43.1%, low: 34.0%, P = 0.03); oxaliplatin (yes: 45.8%, no: 22.9%, P < 0.0001), performance status (PS 0: 46.1%, 1: 28.8%, 2: 26.7%, P < 0.0001), and haemoglobin levels (> or = 12 g dl(-1): 40.4%, < 12 g dl(-1): 29.2%, P = 0.004). In subgroup analysis significant differences in response rate between anaemic and nonanaemic patients were recorded in those patients treated with infusional chemotherapies (45.7 vs 25.5%, P < 0.0001), with high 5FU dose intensity (50.3 vs 32.7%, P = 0.005), with PS = 0 (49.8 vs 37.9%, P = 0.03), and with liver metastases (44.8 vs 33.8%, P = 0.002), whereas no difference was evident in those subjects treated with bolus schedules or according to gender. Anaemia was a strong predictor for activity of first-line 5FU-based chemotherapy especially in those groups that showed the best responses, for example high performance status, infusionally treated, higher 5FU dose and those with liver secondaries. Patients with higher haemoglobin levels recorded a greater response rate and a longer time to progression and survival than anaemic subjects. Prospective evaluation of role of correcting anaemia on response to therapy is justified by these results.