In a Phase I study fosquidone was administered to 23 patients (15 female, 8 male; mean age 53.6 years) with various solid tumours. Increasing doses (50-325 mg/m2) were given intravenously three times a week for 3 weeks. The MTD was established a 325 mg/m2 dose, the dose limiting factor being phlebitis in two of three patients. The other main side-effects were headache (9 patients), pain (11 patients) and nausea and vomiting (16 patients). The majority of these were mild (WHO grade 1-2), but two patients reported WHO grade 3 and 4 pain. There was no clear relationship between dose, severity and frequency of side-effects. Four tumour regressions (three minor and one partial response) were achieved at varying dose levels (50, 100, 160 and 325 mg/m2) in patients with different tumours (laryngeal, colorectal and lung). The recommended dose for Phase II studies is 250 mg/m2 given intravenously three times a week for 3 week.