Preclinical studies demonstrate anti-angiogenic activity of low doses of chemotherapy; selective cox-2 inhibitors are also inhibitors of angiogenesis. Animal data indicates the presence of circulating endothelial cells (CEC), tumor-derived activated endothelial cells (AEC) and endothelial cell progenitors (ECP). Bone marrow-derived ECP have been shown to be incorporated into tumor vasculature. We conducted two combination Phase I studies of celecoxib with either cyclophosphamide or etoposide. Exploratory correlative studies were performed to evaluate the detectability of CEC, AEC and ECP in patients treated with these anti-angiogenic combinations. Patients were treated with oral cyclophosphamide at 50 mg daily or etoposide at 50 mg daily. Celecoxib was given at 400 mg twice daily. Blood samples were collected on days 0, 7, 28 and monthly until disease progression. Blood from healthy volunteers was collected on days 0 and 28. Peripheral mononuclear cells (PMNC) were isolated and stained with fluorescent antibodies and analyzed utilizing 5-color flow cytometry. Forty-four heavily pretreated patients (20 F; 24 M) with various solid tumors were enrolled. Median age was 65 (23-72). Therapy was well tolerated. No responses were seen. Six patients had stable disease for at least 16 weeks. The longest duration on therapy is 420 days in a patient with metastatic thymoma. Pre-therapy CEC were detected in cancer patients and normal controls with mean concentrations of 0.47 cells/uL and 0.14 cells/uL, respectively. Mean ECP in patients and controls were 0.09 cells/uL and 0.03 cells/uL, respectively. No AEC were detected. No consistent changes were seen in CEC or ECP during therapy. The combinations of oral cyclophosphamide or etoposide at 50 mg daily with celecoxib at 400 mg twice daily are well tolerated with occasional prolonged disease stabilizations observed. CEC and ECP are detectable in cancer patients but their levels did not change significantly during therapy with our regimen. Further evaluation of CEC and ECP in patients treated with clinically more active anti-angiogenic therapies would be of interest.