Eighty-six patients with refractory chronic low back pain were randomly assigned to receive either facet joint injection or facet nerve block, using local anaesthetic and steroid. There was no significant difference in the immediate response. The duration of response after facet joint injection was marginally longer than after facet nerve block (P less than 0.05 1 month after infiltration), but for both groups the response was usually short-lived; by 3 months only 2 patients continued to report complete pain relief. Patients who had complained of pain for more than 7 years were more likely to report good or excellent pain relief than those with a shorter history (P less than 0.005), but no other clinical feature was of value in predicting the response to infiltration. Facet joint injections and facet nerve blocks may be of equal value as diagnostic tests, but neither is a satisfactory treatment for chronic back pain.