Matrix metalloproteinases (MMPs) are a family of enzymes that break down extra cellular matrix proteins during tissue formation. Tumours have been shown to over-express certain matrix metalloproteinases relative to normal tissue. Matrix metalloproteinases are associated with the disruption of tissue architecture in the growth of tumours and they are also important in the development of metastases. Pre-clinical studies have shown that inhibitors of matrix metalloproteinases (MMPIs) can restrict malignant growth and block the process of tumour neovascularisation. Marimastat is a synthetic MMPI which has been investigated in clinical studies of patients with advanced cancers.The aim of this open study is to describe the change in serum CEA in eight patients with advanced colorectal metastatic disease treated with Marimastat.The mean percentage change in serum CEA level of the patients before commencing Marimastat treatment was 35.6 (SD 25.7) with a mean percentage change in serum CEA of 6.8 (SD 18.3) during the first month of treatment. The average percentage change in serum CEA after one month of treatment with Marimastat was 20 percent.Although the study sample is small, there is a significant difference in serum CEA change prior to and post commencement of marimastat treatment. It is important to note that the observation period was short for most patients due to their advanced disease.