Six patients undergoing paravertebral blocks for chronically painful conditions of the chest wall were thermographically imaged so that the extent of cutaneous vasodilatation and hence sympathetic block could be correlated with the distribution of the somatic block. All blocks were performed by a single experienced operator, with a single percutaneous entry, using 15 ml of 0.5% bupivacaine at a mean level of T9-10 (range T7-8--T10-11), with radiological confirmation of correct needle placement. There was a mean distribution of the somatic block of five dermatomes (range 1-8), as evidenced by loss of pinprick sensation, with upper and lower limits of T6 and L3. The mean distribution of the sympathetic block was eight dermatomes (range 6-10), as evidenced by ipsilateral skin warming (p = 0.0005-0.001), with upper and lower limits of T5 and L3. No bilateral spread was observed. No significant postural changes in blood pressures were seen, although there was a small but significant decrease in supine heart rate (p = 0.05). This study demonstrates that a large unilateral somatic and sympathetic block is obtainable with a single thoracic percutaneous paravertebral injection. It challenges the suggestions that this method of analgesia is ineffective and hazardous, that a sympathetic component is a rare accompaniment and that the lumbar nerve roots are spared.