BACKGROUND: Acceleromyography (AMG) is increasingly being used in neuromuscular research, including in studies establishing the potency of neuromuscular blocking and reversal agents. However, AMG is insufficiently validated for use interchangeably with the gold standard, mechanomyography (MMG) for this purpose. The aim of this study was to compare AMG and MMG for establishing dose-response relationship and potency, using rocuronium as an example. METHODS: We included 40 adult patients in this randomized-controlled single-dose response study. Anaesthesia was induced and maintained with propofol and opioid. Neuromuscular blockade was induced with rocuronium 100, 150, 200 or 250 microg/kg. Neuromuscular monitoring was performed with AMG (TOF-Watch SX) with pre-load (Hand Adapter) at one arm and MMG (modified TOF-Watch SX) on the other, using 0.1 Hz single twitch stimulation. Dose-response relationships were determined for both recording methods using log (dose) against probit (maximum block). The obtained slopes of the regression lines, ED(50), ED(95) and the maximum block were compared. RESULTS: The ED(50) and ED(95) [95% confidence interval (CI)] for AMG were 185 microg/kg(167-205 microg/kg) and 368 microg/kg(288-470 microg/kg), compared with 174 microg/kg(159-191 microg/kg) and 338 microg/kg(273-418 microg/kg) for MMG. There were no statistically significant biases in maximum block, ED(50), ED(95) or slopes obtained with the two methods. CONCLUSION: Our results indicate that any possible difference between AMG and MMG is so small that it justifies AMG to be used for establishing the potency of neuromuscular blocking agents. However, the wide CIs show that we cannot rule out a 13% higher ED(50) and a 26% higher ED(95) for AMG.