The present invention relates to the treatment of cancer with compounds that inhibit the activity of endo-exonuclease. Endo-exonuclease has been shown to be necessary for the repair of damaged DNA. Compounds that inhibit the activity of endo-exonuclease have been shown to be particularly effective for treating cancer when used in combination with drugs that induce DNA breaks such as cisplatin and mitomycin C. These compounds have a synergistic effect when used in combination for inhibiting tumour growth. The invention includes pharmaceutical compositions for inhibiting tumour growth comprising a compound that inhibits endo-exonuclease activity. These pharmaceutical compositions preferably include compounds that induce DNA breaks. The invention includes methods of treating cancer with these pharmaceutical compositions and uses of these compositions to treat cancer. The preferred compounds that inhibit the activity of endo-exonuclease have low toxicity. One such compound is pentamidine. The invention also includes a method for diagnosing cancer and monitoring its progression. This aspect of the invention involves isolating serum from a patient; measuring the concentration of endo-exonuclease in said serum and determining whether said concentration is above a predetermined mean.