Phosphoramidate derivatives of 5 fluoro 2 deoxyuridine are disclosed for use in the treatment of cancer especially in the treatment of cancer where the patient shows resistance for example in a patient with cells with a lowered level of nucleoside transporter proteins and/or with nucleoside kinase deficient cells and/or with mycoplasma infected cells and/or with cells with a raised level of thymidylate synthase.