A new continuous noninvasive blood pressure measurement device, the T-Line® Tensymeter TL100 (Tensys Medical Inc., San Diego, CA, USA) which enables avoidance of arterial cannulation has been developed. We compared the values obtained using the T-Line values to simultaneous arterial line measurements in patients undergoing general anaesthesia with induced hypotension. Twenty-five patients, aged 18-70 years, were studied. The T-Line and arterial line were positioned on the contralateral wrists. Intra-operative, real-time, blood pressure data were electronically captured and stored on a computer. Bland-Altman plots and 95% limits of agreement show that the majority of T-Line data points were within 5 mmHg of the arterial line measurements (67%) and agreement was within 15 mmHg in 94.6% or more of all measurements. There was virtually no distinguishable error over the course of surgery using the device. In conclusion, the T-Line measurements correlate with arterial measurements during anaesthesia in which there were periods of both normotensive and hypotensive anaesthesia. The T-Line Tensymeter represents a noninvasive alternative to an arterial line in cases when arterial blood sampling is not required.