Inclusion Criteria:

1. Patients with histologically proven supratentorial malignant WHO grade IV gliomas will be eligible for this protocol. These include glioblastoma multiforme (GBM) and gliosarcoma.
2. The patient must have contrast enhancement documented on MRI or CT associated report.
3. Understand and voluntarily sign an informed consent form.
4. Karnofsky performance status of 60 or greater at study entry.
5. Able to adhere to the study visit schedule and other protocol requirements.
6. No more than 16 weeks from the diagnosis of glioblastoma, which is defined as the date of the surgical procedure establishing the histologic diagnosis operation.
7. No more than 1 recurrence of tumor following initial diagnosis.
8. Patients having undergone recent resection of recurrent or progressive tumor will be eligible as long as all of the following conditions apply: 1). At least 1 week has elapsed since the operation. 2). Any blood products visible on brain imaging (CT or MRI) are documented by the treating clinician or radiology report as residual and not active bleeding.
9. Laboratory test results within these ranges: 1). The following two laboratory studies should be performed within14 days from enrollment if receiving cytotoxic chemotherapy; </= 90 days otherwise. (a) Platelet count greater than or equal to 50 x 10^9/L. (b). Hematocrit greater than or equal to 29.0.
10. (10. continued) 2) For the following two laboratory studies, any documented prior normal value is acceptable (including outside institution results) provided that the patient is not taking anticoagulants such as coumadin. If not available, they should be checked. (a) Creatinine less than or equal to 1.5. (b) International Normalized Ratio less than or equal to 1.3
11. Age 18 years or greater at the time of signing the informed consent form. Background data regarding VTE is from adults and may not be applicable to children.
12. This study was designed to include women and minorities, but was not designed to measure differences of intervention effects. Patients will be recruited with no preference to gender.

Exclusion Criteria:

1. Patient is unable to provide informed consent.
2. Pregnant or lactating females, as aspirin may impart addition risk for this patient population.
3. Any condition, including the presence of laboratory abnormalities, which places the subject at unacceptable risk if he/she were to participate in the study or confounds the ability to interpret data from the study as determined by the enrolling physician.
4. Known hypersensitivity or allergy to aspirin.
5. Clinical indication or use of agents with potential of increased bleeding risk via alteration of coagulation pathways or platelet activation including (but not limited to) warfarin, heparins, aspirin, dipyridamole, celecoxib, NSAIDs and clopidogrel. (1) Any use of warfarin, heparinoids, dipyridamole, clopidogrel for greater than 2 consecutive weeks in the prior 6 months (2) Occasional use of NSAIDs, aspirin, or COX-2 inhibitors is not an exclusion if taken on an "as needed" basis less than once per week on average.
6. Diagnosed or suspected peptic ulceration within the last 5 years
7. History of gastrointestinal bleeding within the last 5 years.
8. History of major bleeding (NCIC grade 3-4) within the last 5 years.
9. Hereditary coagulopathy or hypercoagulable state.
10. Anticipated refusal of blood products or maximal supportive care
11. History of spontaneous (non-surgical) intracranial hemorrhage during lifetime.
12. Active or recent uncontrolled gastrointestinal symptoms such as nausea, vomiting, and diarrhea, which are unresolved or within 2 weeks of resolution, or are anticipated to recur.
13. Patient who would be unlikely or unwilling to follow-up at MD Anderson at or more frequently than every 3 months.
14. No exclusion to this study will be based on race. Minorities will actively be recruited to participate. The malignant glioma patient population treated at MDACC over the past year is as follows: American Indian or Alaskan Native - 0. Asian or Pacific Islander - <2%. Black, not of Hispanic Origin - 3%. Hispanic - 6%.

White, not of Hispanic Origin - 88%. Other or Unknown - 2%. Total-100%