The purpose of this clinical research study is to assess efficacy and safety of 3 doses of apixaban 5 mg twice a day, 10 mg twice a day and 20 mg once daily versus conventional treatment with low molecular weight heparin or fondaparinux and vitamin K antagonist in the treatment of subjects with acute symptomatic deep-vein thrombosis.

Inclusion Criteria:

1. Subjects must be willing and able to give written informed consent.
2. Confirmed acute symptomatic DVT, i.e., proximal vein or extensive calf-vein thrombosis, involving at least the upper third part of the deep calf veins (trifurcation area) without concomitant symptomatic PE.
3. Women and men, ages 18 (or legal age of consent) to 90. Women of childbearing potential (WOCBP) must be using an adequate method of contraception to avoid pregnancy throughout the study and for up to 1 week after the study in such a manner that the risk of pregnancy is minimized.

WOCBP include any female who has experienced menarche and who has not undergone successful surgical sterilization (hysterectomy, bilateral tubal ligation or bilateral oophorectomy) or is not postmenopausal [defined as amenorrhea for 12 consecutive months; or women on hormone replacement therapy (HRT) with documented serum follicle stimulating hormone (FSH) level > 35mIU/mL]. Even women who are using oral, implanted or, injectable contraceptive hormones or mechanical products such as an intrauterine device or barrier methods (diaphragm, condoms, spermicides) to prevent pregnancy or practicing abstinence or where partner is sterile (e.g., vasectomy), should be considered to be of child bearing potential. WOCBP must have negative serum or urine pregnancy test (minimum sensitivity 25IU/L or equivalent units of HCG) within 24 hours prior to the start of study medication.

Exclusion Criteria:

1. Women who are pregnant or breastfeeding.
2. Women with a positive pregnancy test on enrollment or prior to study drug administration.
3. More than 24 hours pre-randomization treatment with therapeutic dosages of unfractionated heparin (UFH), low molecular weight heparin (LMWH) or fondaparinux or more than a single starting dose of vitamin K antagonist (VKA) prior to randomization.
4. Uncontrolled hypertension: systolic blood pressure > 200 mm Hg or diastolic blood pressure > 110 mm Hg.
5. Creatinine clearance < 30 mL/min
6. Impaired liver function (ALT > 3 x ULN)
7. Use of ASA > 165 mg/day
8. WOCBP who are unwilling or unable to use an acceptable method of birth control to avoid pregnancy for the entire study period and for up to 1 week after the study.
9. Azole antifungals (e.g., ketoconazole), HIV protease inhibitors (e.g., ritonavir) and macrolide antibiotics (e.g., erythromycin).

NOTE: topical azole antifungal agents are permitted.