The T-Line TL-200 (Tensys Medical, San Diego, CA, USA) is a non-invasive arterial blood pressure (BP) monitoring system allowing continuous "beat-to-beat" monitoring of systolic arterial pressure (SAP), mean arterial pressure (MAP), and diastolic arterial pressure (DAP). It provides a real-time BP waveform like that obtained using an arterial catheter. The aim of this study was to compare BP measurements obtained using the T-Line TL-200 with simultaneous invasive BP measurements using a femoral arterial catheter in unselected critically ill medical patients. In 28 patients treated in a medical intensive care unit (ICU), BP values were simultaneously obtained using a femoral arterial catheter and the T-Line TL-200. All recorded data were included in the final analysis. For comparison of BP measurements, Bland-Altman analysis accounting for repeated measurements was performed (primary endpoint). A total of 76,826 pairs of BP measurements (each consisting of SAP, MAP, and DAP) were analyzed. For MAP, Bland-Altman analysis revealed a mean difference of +0.47 mmHg (95 % limits of agreement -16.53 to +17.46 mmHg). For SAP and DAP, the bias and 95 % limits of agreement were -9.01 mmHg (-37.47 to +19.45 mmHg) and +5.22 mmHg (-13.50 to +23.94 mmHg), respectively. Non-invasive, continuous, radial BP measurement with the T-Line TL-200 is basically feasible in medical ICU patients (with a low bias for MAP compared to MAP assessed using a femoral arterial catheter). High limits of agreement (particularly of SAP and DAP) preclude the use of the device as a single source of BP information in unstable critically ill patients.