PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No concurrent methotrexate Endocrine therapy: Not specified Radiotherapy: Not specified Surgery: See Disease Characteristics No prior small bowel resection Other: No concurrent anticonvulsants No concurrent drugs that markedly interfere with folate absorption or metabolism (e.g., sulfasalazine and phenytoin) No concurrent vitamins during and for 2 years after beginning of study