Abstract: The invention relates to pharmaceutical compositions intended to decrease the incidence of tumor metastasis in patients who suffer from malignant diseases.The pharmaceutical compositions of the invention contain as active ingredient heparin or a suitable derivative thereof. Amongst suitable derivatives are N-desulfated and N-acetylated heparin.The dosage of the administered heparin or heparin derivative is quite critical and will generally be in the range of from 0.05 mg/kg/day to about 0.5 mg/kg/day. A preferred range is between about 0.1 mg/kg/day to about 0.5 mg/kg/day.