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We have developed an easy-to-use, noninvasive piezoelectric sensor for quantitative monitoring of neuromuscular block. In a clinical evaluation with 23 patients, the piezo sensor was objectively compared to a mechanomyogram (MMG) for its ability to measure train-of-four (TOF) ratio from the adductor pollicis. After administration of succinylcholine (120–200 mg intravenously [IV]) to facilitate intubation, neuromuscular block was maintained with vecuronium by either boluses (1–2 mg IV) or an infusion (0.4–1.0 μg·kg−1.min−1 IV). Paired measurements were made of the TOF ratio from both sensors over a complete range of block levels (8%-100%). The difference in the TOF ratio measurement between the sensors showed a bias of 0.018. The SD of the difference between the sensors was ± 0.129. The limits of agreement, which define the range in which 95% of the differences between the sensor measurements lie, were from-0.24 to 0.275. The sensitivity of the piezo sensor for detecting recovery based on a TOF ratio greater than 0.70 was shown to be 0.74 with specificity of 0.91. Under the conditions tested, the piezo sensor was not as accurate as the MMG. However, it was able to predict recovery of neuromuscular block with better accuracy than shown previously by manual evaluation of the TOF ratio, making it a reasonable, convenient alternative for quantitative monitoring of recovery from neuromuscular block.