SU6668 is a small molecule inhibitor of the angiogenic receptor tyrosine kinases Flk‐1/KDR, PDGFRβ, and FGFR1. In mice, SU6668 treatment resulted in regression or growth arrest of all large established human tumor xenografts examined associated with loss of tumor cellularity. The events underlying loss of tumor cellularity were elucidated in detail in several tumor models. SU6668 treatment induced apoptosis in tumor microvessels within 6 h of the initiation of treatment. Dose‐dependent decreases in tumor microvessel density were observed within 3 days of the first treatment. These changes were accompanied by decreased tumor cell proliferation and increased tumor cell apoptosis. Rapid increases in VEGF transcript levels were seen, consistent with the induction of tumor hypoxia. Using Western blot analyses, we determined that these in vivo antiangiogenic and proapoptotic effects of SU6668 occur at doses comparable to those required to inhibit Flk‐1/KDR and PDGFRβ phosphorylation in tumors. Potent, dose‐dependent inhibition of Flk‐1/KDR activity in vivo was independently demonstrated using vascular permeability as a readout. These data demonstrate that SU6668‐induced inhibition of angiogenic receptor tyrosine kinase activity in vivo is associated with rapid vessel killing in tumors, leading to broad and potent antitumor effects.