Abnormalities of 5 cyclooxygenase products were studied in tumours from patients with head and neck cancer. The studies were designed to measure tissue prostaglandin content (ng/g wet tissue) as well as production in vitro by tumour microsomes (ng/mg protein/10 min). Head and neck tumours contained large amounts of 5 eicosanoids in the order. PGE2 greater than PGE1 greater than 6-keto-PGF1 alpha greater than PGF2 alpha greater than TXB2. When tumours less than or equal to 2 cm were compared with tumours equal to or more than 3 cm in size, amounts of all but PGF2 alpha were higher in the larger tumours. However, the capacity of tumour microsomes to synthesize PGs in vitro was inversely related to tumour size. These results suggest a defect in removal of eicosanoids from large tumours. The physiological significance of this increase in local levels of eicosanoids remains to be determined in head and neck cancer.