Raltitrexed (ZD-1694) is a quinazoline-based folate analogue that exerts its cytotoxic activity by the specific inhibition of thymidylate synthase. In vitro studies show that raltitrexed is actively transported into cells and is then rapidly and extensively metabolised to a series of polyglutamates. These metabolites are significantly more potent inhibitors of thymidylate synthase than the parent drug and are retained intracellularly, producing prolonged cytotoxic effects without the need for continuous drug exposure. Phase III clinical trials in patients with advanced colorectal cancer evaluated raltitrexed 3 mg/m2 administered as a 15-minute intravenous infusion once every 3 weeks. This schedule produced objective response rates of 14.3 to 19.3%, which were similar to those in patients treated with fluorouracil plus leucovorin (15.2 to 18.1%). Median survival durations ranged from 9.7 to 10.9 months with raltitrexed treatment and from 10.2 to 12.7 months with fluorouracil plus leucovorin. The major toxicities associated with raltitrexed involve the haematological and gastrointestinal systems, although severe asthenia also occurred in 6 to 18% of patients receiving the drug. Grade 3 or 4 nausea or vomiting occurred in up to 13% of raltitrexed recipients and grade 3 or 4 diarrhoea in up to 14%. Similar incidences of grade 3 or 4 nausea or vomiting and diarrhoea were seen with fluorouracil plus leucovorin treatment. Raltitrexed generally showed significant advantages over fluorouracil plus leucovorin with respect to the incidence of leucopenia and mucositis. A greater proportion of raltitrexed than fluorouracil plus leucovorin recipients were able to receive the scheduled dosage. Thus, with its similar efficacy to fluorouracil-based regimens, convenient administration schedule and favourable tolerability profile, raltitrexed provides clinicians with a worthwhile alternative to fluorouracil-based treatment for patients with advanced colorectal cancer.