e16529 Background: Up to 17% of women with ovarian cancer (OC) develop venous thromboembolism (VTE). LMWH reduces the risk of VTE and may have antineoplastic effects because tissue factor on OC cells stimulates tumor growth. A multicentre, open-label phase II randomized trial was undertaken to identify a potentially efficacious and safe dose of dalteparin to investigate its antineoplastic potential in OC. Methods: Women with newly diagnosed epithelial OC were randomized to receive taxane and platinum chemotherapy (CT) and one of 3 doses of dalteparin (50 IU/kg, 100 IU/kg, or 150 IU/kg) subcu daily during the first 3 cycles of CT. With each cycle of CT, patients had a CA125 and were assessed for bleeding or VTE. A response was defined as a ≥50% reduction in CA125 from baseline sustained for at least 28 days (according to GCIG criteria). To select the best LMWH group with a ≥ 80% response rate (RR) {vs. 60% with CT alone} with 90% probability, 39 patients per group were needed. Patients were followed until the end of CT. Results: 77 women were randomized and 72 of them completed follow-up. Compliance with injections was > 80%. There were no major bleeding events and 3 patients in the highest dose group had minor bleeding. There were no VTEs while on or within 7 days of finishing LMWH. There were 2 deaths attributed to the underlying OC. A 50% drop in CA125 by the start of cycle 4 was seen in 87% of the 50 IU/kg group, 92% of the100 IU/kg group, and 84% of the 150 IU/kg group, respectively. The corresponding RRs by the end of cycle 6 were 91%, 92% and 92%. Conclusions: Use of dalteparin is safe and well tolerated in women who are receiving CT for newly diagnosed OC. This therapy was effective in preventing VTE. A dose response effect on CA125 levels was not identified. [Table: see text]