We investigated performance of an arterial tonometer during anesthesia in the elderly. Thirty patients (17 men and 13 women), ranging from 70 to 86 years old, were included, and agreement between tonometric arterial pressure (TAP) and intraarterial pressure (IAP) was calculated. A total of 6487 paired points was sampled, and values for precision (mean absolute difference +/- SD) were 6.8 +/- 5.2 for systolic, 6.9 +/- 4.6 for mean, and 9.2 +/- 5.4 mmHg for diastolic blood pressures. Values for bias and limits of agreement [mean difference (TAP - IAP) +/- SD, mean difference (TAP - IAP) +/- 2 SD, respectively] for systolic, mean, and diastolic pressure were 1.2 +/- 8.4 and 1.2 +/- 16.8, 5.7 +/- 5.9 and 5.7 +/- 11.8, and 8.6 +/- 6.2 and 8.6 +/- 12.4 mmHg, respectively. Compared with previous data, aging is likely to affect the performance of an arterial tonometer.