Clinical and experimental studies have suggested that unfractionated heparin (UFH) effects malignancy progression. We reviewed all published clinical reports concerning the effects of UFH, as compared to no treatment on survival of cancer patients. Studies were classified on methodological strength and subdivided as to whether therapeutic or prophylactic dosages of UFH were used. Mortality rates after 3 years were extracted or calculated. One randomized study that evaluated the use of UFH in therapeutic dosages in patients with small cell lung carcinoma reported on an improved survival (odds ratio (OR) 0.64; 95% confidence interval (CI): 0.25 to 1.62). A detrimental effect was observed in 2 randomized studies which investigated the effects of intraportal UFH treatment in a prophylactic dose after surgery for gastrointestinal cancer (OR 1.66; 95% CI: 1.02 to 2.71). In contrast, level 2 studies in which either therapeutic or prophylactic dosages of UFH on mortality of patients with gastrointestinal cancer were evaluated, showed OR of 0.58 (95% CI; 0.11-3.13) and 0.65 (95% CI 0.51 to 0.84), respectively. We conclude that there is no convincing evidence of either positively or negatively effects of UFH on survival of patients with malignancy.