DISEASE CHARACTERISTICS: Histologically confirmed advanced adenocarcinoma of the stomach or gastroesophageal junction beyond hope of surgical cure and not considered a candidate for potentially curative chemotherapy/radiation therapy Measurable or evaluable disease Tumor must be accessible for biopsy No known central nervous system metastases or carcinomatous meningitis

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: ECOG 0-2 Life expectancy: At least 12 weeks Hematopoietic: Absolute granulocyte count at least 1,500/mm3 Platelet count at least 150,000/mm3 Hemoglobin at least 9.0 mg/dL (transfusion allowed) Hepatic: Bilirubin no greater than upper limit of normal (ULN), regardless of liver involvement secondary to tumor AST no greater than 3 times ULN Renal: Creatinine no greater than 1.5 times ULN Cardiovascular: No unstable angina No uncontrolled high blood pressure No active congestive heart failure No myocardial infarction in the last 6 months No serious uncontrolled cardiac arrhythmia No New York Heart Association Class III or IV heart disease Pulmonary: No interstitial pneumonia or extensive and symptomatic interstitial fibrosis of the lung No pleural effusion or ascites, which cause respiratory compromise (at least Grade 2 dyspnea) Other: No active or uncontrolled infection No prior malignancy, except for adequately treated basal cell or squamous cell skin cancer, adequately treated noninvasive carcinomas, or other cancer from which the patient has been disease free for at least 5 years Not pregnant or nursing Adequate contraception required of all fertile patients

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy for gastric or gastroesophageal junction cancer No prior irinotecan (CPT-11) or other camptothecin Endocrine therapy: Not specified Radiotherapy: No prior abdominal or pelvic radiotherapy No prior radiotherapy to greater than 25% of bone marrow No prior radiotherapy to measurable or evaluable indicator lesions At least 4 weeks since major radiotherapy (chest radiotherapy) Surgery: At least 3 weeks since major surgery and recovered At least 2 weeks since minor surgery and recovered Other: No other concurrent investigational agents