Fifty-two patients, admitted to a pain relief unit, had a cannula placed in the radial artery to measure the paO2, paCO2 and pH of arterial blood every 2 h, for periods ranging from 12 to 24 h. The patients were divided into 3 groups: 14 had low back pain, 21 patients had pain from cancer, and 17 had pain from other causes. Twenty were male and 32 were female with a mean age of 53 years (range 16-82 years). The mean paO2 of these groups was within normal limits. The mean paCO2 and pH for the 3 groups were, low back pain paCO2 4.1 kpa, pH 7.42, others, paCO2 4.2 kpa, pH 7.42. The finding of a normal pH associated with a low paCO2 suggests that patients were "reset" to a low paCO2. Treatment, which was most commonly nerve blocks, resulted in marked pain relief in 30 patients. Ten of these patients were available for follow-up at least 1 week later (4 from the low back pain group, 6 from the cancer group), and in every patient, after pain relief, there was a rise in paCO2 which was statistically significant (P less than 0.001) and was not accompanied by a fall in pH. This suggests that intractable pain is accompanied by chronic hyperventilation and that the relief of pain is accompanied by a decrease in ventilation.