PRIOR CONCURRENT THERAPY: Biologic therapy: No concurrent colony-stimulating factors during the first course of study Chemotherapy: More than 4 weeks since prior chemotherapy No more than 1 prior chemotherapy regimen for carcinoma of the esophagus or gastric cardia No prior fluorouracil Endocrine therapy: Not specified Radiotherapy: At least 4 weeks since prior radiotherapy Surgery: Not specified Other: No other concurrent investigational agents No concurrent antiretroviral therapy in HIV-positive patients No other concurrent anticancer agents