Inclusion Criteria:

- Histologically confirmed diagnosis of adenocarcinoma of the gastric. Stage IV
disease, not amenable to curative surgery, or disease recurrence after prior surgery.

- Disease status must be that of measurable disease with presence of at least one
measurable lesion as defined by Response Evaluation Criteria in Solid Tumors (RECIST)
criteria.

- In bidimensionally measurable lesions the longest diameter should be selected for
measurement.

- Tumor lesions in areas of prior radiation therapy may be included only if they were
clearly progressing.

- If only a single lesion is present in a patient who had prior therapy for gastric
adenocarcinoma, the neoplastic nature of the lesion should be confirmed by cytology
and/or histology.

- Ultrasound and clinical examination are not allowed for assessment of measurable
disease.

- Elevation of tumor markers, pleural or pericardial effusion, ascites, bone lesions,
cystic lesions, or carcinomatous lymphangitis pulmonis/cutis is defined as not being
measurable.

- Performance Status of 0 or 1 on the Eastern Cooperative Oncology Group (ECOG) Scale.

- Estimated life expectancy of at least 12 weeks.

- No prior chemotherapy or radiotherapy.

- Patient compliance and geographic proximity that allow adequate follow-up.

Adequate organ function including the following:

- Bone marrow: absolute neutrophil count 1.5 x 10 to the ninth power/liter (L),
platelets 100 x 10 to the ninth power/L, hemoglobin >=9 grams per deciliter (g/dL).

- Hepatic: bilirubin <=1.5 x upper limit of normal (ULN); alkaline phosphatase,
aspartate transaminase and alanine transaminase <=3.0 x ULN.

- Renal: Calculated creatinine clearance >=45 milliliters (ml)/minute.

- Men or women, age 18 to 70 years.

- For women: Must be surgically sterile, post-menopausal, or compliant with a
medically approved contraceptive regimen during and for 3 months after the treatment
period; must have a negative serum or urine pregnancy test within 7 days before study
enrollment and must not be breast-feeding.

- For men: Must be surgically sterile, or compliant with a contraceptive regimen
during and for 3 months after the treatment period.

- Signed informed consent from patient.

Exclusion Criteria:

- Have received treatment within the last 30 days with a drug that has not received
regulatory approval for any indication at the time of study entry.

- Concurrent administration of any other tumor therapy.

- Active infection.

- Serious concomitant disorders that would compromise the safety of the patient or
compromise the patient's ability to complete the study, at the discretion of the
investigator.

- Pregnancy.

- Breast-feeding.

- History of significant neurological or mental disorder, including seizures or
dementia.

- Have had a prior malignancy other than gastric cancer, carcinoma in situ of the
cervix, or nonmelanoma skin cancer, unless that prior malignancy was diagnosed and
definitively treated at least 5 years previously with no subsequent evidence of
recurrence.

- Patients with a history of low grade (Gleason score less than or equal to 6)
localized prostate cancer will be eligible even if diagnosed less than 5 years
previously.

- Inability to interrupt aspirin or other nonsteroidal anti-inflammatory drugs 2 days
before, the day of, and 2 days after the dose of pemetrexed plus cisplatin.

- If a patient is taking a nonsteroidal anti-inflammatory drug (NSAID) or salicylate
with a long half-life it should not be taken 5 days before, the day of, and 2 days
after the dose of pemetrexed plus cisplatin.

- Clinically significant ascites or pleural effusion that is apparent at clinical
examination and cannot be controlled by drainage or other procedures prior to study
enrollment. NOTE: Small effusions noted on computed tomography (CT) scan do not
exclude the patient from study enrollment.

- Inability or unwillingness to take folic acid, vitamin B12 supplementation, or
dexamethasone.

- Known or suspected brain metastasis. Patients who have clinical signs or symptoms
that are suspicious of brain metastasis must have a pretreatment CT or magnetic
resonance imaging (MRI) of the brain. A patient with documented brain metastasis, at
the time of consideration for study entry or in the past, will be excluded from
entering in the study.