A new multisize blood pressure cuff (TriCuff; Inmed) which automatically adjusts to fit any size arm was tested against the routinely used 12 x 23 cm cuff in patients with an arm circumference less than 33 cm (group A = 30) and against the 15 x 33 cm cuff as well as the 12 x 23 cm cuff in patients with arm circumference greater than 33 cm (group B = 26). Arm circumference in group A was 25 +/- 3 cm and in group B 38 +/- 3 cm (P less than 0.01). Systolic blood pressure (SBP) and diastolic blood pressure (DBP) in group A was 125 +/- 20 mmHg and 78 +/- 11 mmHg respectively with the TriCuff and 124 +/- 19 mmHg and 78 +/- 13 mmHg with the 12 x 23 cm cuff. In group B SBP and DBP with the TriCuff was 134.6 +/- 13.3 mmHg and 84 +/- 17 mmHg respectively compared with 132 +/- 15 mmHg and 83.8 +/- 16 mmHg measured by the 15 x 33 cm cuff. SBP and DBP with the 12 x 23 cm cuff was 139.8 +/- 16 mmHg and 90.8 +/- 14 mmHg respectively. Both these values are significantly higher (P less than 0.05) compared with the new cuff and the larger cuff. Using the 12 x 23 cm cuff in group B resulted in the false diagnosis of hypertension in 3 normotensive patients. The TriCuff gives accurate blood pressure measurements in patients with different arm circumferences and should prevent false-positive diagnoses of hypertension in obese patients.