Antiangiogenesis therapy for cancer may inhibit tumor growth and metastasis when combined with chemotherapy, and has received a great deal of attention over recent years. However, accurate assessments of biological efficacy and toxicity are major hurdles for this approach. Soluble VEGF receptor-1 (sFlt-1) has been reported to have a role in the pathogenesis of preeclampsia, the hallmark of which is similar to the toxicities related to antiangiogenesis therapy. Clinical evidence and animal studies support the hypothesis that sFlt-1 may contribute to hypertension and proteinuria in patients treated with anti-VEGF agents. The intratumoral imbalance between sFlt-1 and VEGF levels correlates with the malignancy grades of tumors, survival and responsiveness to therapy. The therapeutic potential of sFlt-1 as an antiangiogenic agent has been validated by an increasing number of preclinical studies. Furthermore, antiangiogenesis therapy changes the concentration of circulating VEGF, PlGF, sFlt-1, soluble VEGFR-2 and even soluble VEGFR-3, with some of these being identified as potential biomarkers of response and toxicity. All these factors suggest that sFlt-1 may prove invaluable for driving the future development of molecular therapeutics with novel targets and mechanisms of action, and its impact on antiangiogenesis therapy in cancers needs further investigation.