We measured carotid and brachial artery blood flow by Doppler ultrasound in 11 human volunteers, and related these to cardiac index and to each other. The median (IQR [range]) carotid arterial blood flow was 0.334 (0.223–0.381 [0.052–0.563]) l.min−1 on the right and 0.315 (0.223–0.369 [0.061–0.690]) l.min−1 on the left. The brachial arterial blood flow was 0.049 (0.033–0.062 [0.015–0.204]) l.min−1 on the right and 0.039 (0.027–0.054 [0.011–0.116]) on the left. Cardiac index was 3.2 (2.8–3.5 [1.9–5.4]) l.min−1.m−2. There was a moderate to good correlation between right‐and left‐sided flows (brachial: ρ = 0.45; carotid: ρ = 0.567). Brachial and carotid flow had no or a negative correlation with cardiac index (right brachial: ρ = −0.145, left brachial: ρ = −0.349; right carotid: ρ = −0.376, left carotid: ρ = −0.285). In contrast to some previous studies, we found that Doppler‐estimated peripheral arterial blood flows only show a weak correlation with cardiac index and cannot be used to provide non‐invasive estimates of cardiac index in man.