In this phase III, multicenter study, 1200 patients with lung, breast, gastrointestinal (stomach, colon-rectum, pancreas), ovarian or head and neck cancer undergoing chemotherapy will be randomly assigned (at the beginning of cytotoxic therapy) in a 2:1 ratio and in double-blind conditions to a treatment with subcutaneous low molecular weight heparin (nadroparin calcium, one injection/day) or placebo for the overall duration of chemotherapy or up to a maximum of 4 months (+/- 10 days).