The failure rate and complications following thoracic and lumbar paravertebral blocks performed in 620 adults and 42 children were recorded. The technique failure rate in adults was 6.1%. No failures occurred in children. The complications recorded were: inadvertent vascular puncture (6.8%); hypotension (4.0%); haematoma (2.4%); pain at site of skin puncture (1.3%); signs of epidural or intrathecal spread (1.0%); pleural puncture (0.8%); pneumothorax (0.5%). No complications were noted in the children. The use of a bilateral paravertebral technique was found approximately to double the likelihood of inadvertent vascular puncture (9% vs. 5%) and to cause an eight-fold increase in pleural puncture and pneumothorax (3% vs. 0.4%), when compared with unilateral blocks. The incidence of other complications was similar between bilateral and unilateral blocks.