The failure rate and complications were studied prospectively in 367 paediatric and adult patients who had received a thoracic or lumbar paravertebral block. The overall failure rate was 10.1%; adults 10.7%; children 6.2%. The frequency of complications were: hypotension: 4.6%; vascular puncture: 3.8%; pleural puncture: 1.1%; pneumothorax: 0.5%. Since these results are similar to those found with alternative methods, e.g. epidural, intrapleural and intercostal blocks, paravertebral block can be recommended as an effective, safe technique for unilateral analgesia in both adults and children.