Antisense oligonucleotides targeted to sequences in thymidylate synthase (TS) mRNA. In particular the invention relates to antisense oligonucleotides targeted to sequences in the 3' end of TS mRNA, which are both cytostatic on their own when administered to human tumour cell lines, and which also enhance the toxicity of anticancer drugs. The invention further relates to a combination product comprising an antisense oligonucleotide in combination with an anticancer agent such as Tomudex or pemetrexed and to the use of such a combination product in the treatment of cancer.