Osteoid osteoma is a tumour of bone characterised by pain which is relieved by aspirin and nonsteroidal anti-inflammatory drugs. Very high levels of prostaglandins have been found in the lesion. In five patients with osteoid osteoma, prostaglandin E2 (PGE2) and prostacyclin (PGI2) synthesis in the nidus yielded 1155.6 +/- 496.5 (mean +/- SD) and 245.2 +/- 89.8 pg/mg respectively, values which are 33 and 26 times higher than in fragments of normal bone. The sclerotic bone around the nidus produced both prostaglandins at the same rate as normal bone. In three patients the excretion rate of the major urinary metabolite of systemic PGI1 was reduced to 50% one month after removal of the tumour. The urinary excretion rate of 6-keto-PGF1 alpha, reflecting intrarenal PGI2 synthesis, was not changed after operation. These results offer new insight into the pain mechanism in osteoid osteoma.