Eicosanoids are involved in most cellular activities. Measurement of their levels in tissue or blood renders information about the function of activated cells. An extended analysis will improve the conclusions regarding eicosanoid-related diseases. Peripheral white blood cells (WBC) were used for the test. Stimulating or inhibiting substances to influence the generation and the metabolism of eicosanoids were separately added to the samples. Prostaglandins (PG) and leukotrienes (LT) were measured after incubation in culture medium for 20 minutes at room temperature. Healthy controls rendered normal data. Patients with intolerance to acetylsalicylic acid (ASS) showed an elevated output of PG and LT upon stimulation. Addition of ASS shifted from PG to LT. An altered pattern of eicosanoids also was found in patients suffering from gastroduodenal ulcer and in intestinal malignancy. The sensitivity regarding the ASS-intolerance is >80% and the specificity in the same group >70%. We concluded that the FET is a suitable test for the demonstration and verification of intolerance to ASS. It also detects an imbalance of the eicosanoids in intestinal malignancy. This makes the FET a helpful tool for diagnosis and for the elucidation of pathogenic mechanisms.