11002 Background: Recently we have shown that in preclinical models and in cancer patients certain types of chemotherapy can induce an acute release of progenitor cells (Shaked et al, Cancer Cell 2008). In the preclinical models these cells diminished the efficacy of chemotherapy. Little is known of the kinetics of circulating endothelial (progenitor) cells (CE(P)C) release after chemotherapy in humans and their respective temporal functionality. Here we investigated the temporal changes in CE(P)C release after chemotherapy. Methods: Patients with mainly advanced cancer treated with various types of MTD chemotherapy were included. Blood sampling was performed at base line, 4 hours, 7 and 21 days after start chemotherapy. The mononuclear cell fraction was analyzed for CE(P)C as previously described (Duda et al, Nature Protocols 2007) Response to chemotherapy was assessed after 3 cycles of chemotherapy following the RECIST criteria. Progression free survival (PFS) was monitored. Wilcoxon matched pairs test, Spearman correlation and the Kaplan-Meyer method were used. Results: 69 patients were included, 49 were evaluable for the complete data set. In all patients treated with paclitaxel there was an immediate increase in EPC 4 hours after start of treatment (median 246% (p<0.05). Other combinations of chemotherapy induced a decrease in EPC (53%, p < 0.01). These immediate changes did not correlate with response or PFS. At day 7, both CEC and EPC levels correlated with change in tumor volume after 3 cycles of chemotherapy (r 0.4, p<0.05). At day 21 after start of chemotherapy, we found a consistent increase in EPC (315%, p 0.01) and CEC (291%, p 0.01) levels compared to baseline, independent of the chemotherapeutic used or of the presence of a tumor. CEC levels after 7 and 21 days correlated with PFS, regardless of the type of chemotherapy (day 7: HR 0.26 (95%CI 0.05–0.52, p <0.01) and day 21: HR 0.4 (95%CI 0.1–0.9, p<0.05). EPC levels did not correlate with PFS. Conclusions: These findings indicate that the longterm release of CE(P)C is a common phenomenon after treatment with chemotherapy. The correlation with a clinical response provides further support for the biological relevance of these cells.The extend of this release may influence the response to therapy and the prognosis. No significant financial relationships to disclose.