The objective of the study was to try to monitor the nutritional status of cancer patients during chemotherapeutic treatment. Concomitantly with chemotherapeutic treatment administered to patients with cancer of the gastrointestinal tract and metastatic carcinoma of unknown origin, levels of carotene, retinol, thiamine, riboflavin, pyridoxine, iron, total protein and hemoglobin were measured in the blood periodically. In addition, anthropometric studies were performed and the nutritional status was established. A total of 19 patients were subject for final evaluation. These patients formed 3 groups according to their nutritional status (good, medium, poor). The effect of chemotherapy was correlated to the nutritional status at 3 different periods of chemotherapy. Most patients with good clinical status maintained the initial nutritional status. Half of the patients with medium nutritional status improved clinically during therapy, and patients initially with poor nutritional status further deteriorated. The levels of most vitamins decreased to a certain degree during therapy and returned to initial values thereafter. Our impression is that cancer patients might benefit from intensive ongoing personal nutritional monitoring and counselling. The results presented have a preliminary meaning because of the small number of patients included in this study.