A reliable, safe approach to achieving unilateral anesthesia in multiple contiguous thoracic dermatomes would be of great benefit to anesthesiologists in the acute and chronic pain setting. The multidermatomal intercostal technique is one such approach, although the anatomical mechanism of this nerve block is a matter of debate. At our pain clinic, we have used another technique, a modification of the paravertebral block, to achieve multiple segments of unilateral sensory blockade. We have used this technique, which we call the paravertebral-peridural block, for over 20 years in the treatment of various pain problems. In retrospective analysis of the 384 blocks performed from 1982 to 1986, there was one pneumothorax (0.26%). one thecal puncture (0.26%), and two accidental intrathecal injections (0.52%). Eighteen blocks (4.6%) resulted in transient hypotension. There were no permanent sequelae. Ninety-three percent of blocks were evaluated as—or—in quality. Bilateral sensory blockade was documented in five patients (1.3%). In order to clarify the mechanism of bilateral blockade resulting from a unilateral technique. we injected four fresh cadavers with colored latex solution using the paravertebral-peridural approach. This revealed spread of the latex across the midline prevertebrally to the contralateral paravertebral space. We conclude that the paravertebral-peridural thoracic block is a reliable, safe technique for achieving unilateral anesthesia over multiple dermatomes with a single injection.