Cardiac autonomic function was assessed by an on-line microcomputer technique in 67 patients with diabetic retinopathy. Sixteen patients had a resting tachycardia (92 beats/min or greater). R-R interval variations were abnormally low in 29 patients at rest, in 35 during deep breathing and in four in response to a Valsalva manoeuvre. The severity of retinopathy correlated significantly with the degree of autonomic dysfunction assessed by the interval variations at rest (p less than 0.05), during deep breathing (p less than 0.01), and with the Valsalva manoeuvre (p less than 0.01). The results emphasise the important association between diabetic retinopathy and cardiac autonomic dysfunction.