Background: The analgesic effect of a TAP block has been investigated in various surgical settings. There are however limited information about block level and block duration. Furthermore, there is a lack of information about continuous TAP block after ultrasound-guided posterior TAP blocks. The aim of this double-blind randomized study was therefore to investigate the effect of an ultrasound-guided posterior TAP block with 24 hours local anesthetic infusion via a TAP catheter. Methods: In this randomized study 8 male volunteers received a bilateral TAP block (20 mLs 0.5% ropivacaine) and were allocated to receive active infusion (ropivacaine 0.2% 5 mL/hr) via a TAP catheter on one side and placebo infusion on the other side. Primary outcome: Dermatomal sensory block involvement after 24 hours evaluated with pinprick. Secondary outcomes: Sensory block involvement evaluated with cold test and heat-pain detection thresholds (HPDT) on the abdominal wall. Assessment points: 15 min before block performance and 1, 4, 8, 12 and 24 hours after block performance. Results: The TAP block primarily involved sensory changes in the Th10 to Th12 dermatomes. On the placebo side there was a decrease in extension beginning at 4-8 hours after block performance and with no detectable effect beyond 12 hours. Median number of dermatomes anesthetized (pinprick) at 24 hours after block performance was 1.5 (0-3) on the active side compared with 0 (0-0) on the placebo side (P = 0.039).There were no statistical significant between-side differences in HPDT measurements at 24 hours after block performance. Conclusions: The spread of sensory block following ultrasound-guided posterior TAP block is partly maintained by a continuous 24 hour ropivacaine infusion through a TAP catheter. Trial registration: The study was registered at NCT01577940.