The primary purpose of the study is to evaluate the efficacy and safety of temozolomide compared to semustine in the treatment of patients with glioblastoma multiforme or anaplastic astrocytoma.

Inclusion Criteria:

• Prior histologic confirmation of glioblastoma, anaplastic astrocytoma.
• Evidence of tumor progression or recurrence.
• Age >=18 years.
• Karnofsky performance status >=60%.
• Absolute neutrophil count >=1,500/mm^3, platelet count >=100,000/mm^3, hemoglobin >=8g/dL.
• Serum BUN and creatinine <1.5 times upper normal limit of testing laboratory (ULN).
• Total bilirubin and direct bilirubin <1.5 times ULN.
• SGOT, SGPT <3 times ULN; alkaline phosphatase <2 times ULN.
• Life expectancy greater than 3 months.
• Informed consent obtained.
• If palliative radiation is needed, agree to give it prior to initiating chemotherapy with study drug. If palliative radiation is required during treatment with study drug, the patient should be permanently discontinued from further treatment with study drug.
• Women of childbearing potential must use a medically accepted, effective method of contraception.
• Women of childbearing potential must have a negative serum pregnancy test 24 hours prior to administration of study drug.

Exclusion Criteria:

• Chemotherapy (excluding nitrosourea, mitomycin C or vincristine), biologic therapy or immunotherapy within 4 weeks, inclusive, prior to study drug administration.
• Nitrosourea or mitomycin C administration within 6 weeks, inclusive, prior to study drug administration.
• Vincristine within 2 weeks prior to study drug administration.
• Completion of radiation therapy, interstitial brachytherapy or radiosurgery within 4 weeks prior to study drug administration.
• Surgery within 3 weeks, inclusive, prior to study drug administration.
• Acute infection requiring intravenous antibiotics.
• Frequent vomiting or medical condition that could interfere with oral medication intake (eg, partial bowel obstruction).
• Previous or concurrent malignancies at other sites with the exception of surgically cured carcinoma in-situ of the cervix and basal or squamous cell carcinoma of the skin.
• Known HIV positive or AIDS-related illness.
• Pregnant or nursing women.
• Men who are not advised to use an effective method of contraception.