Five different methods of analysing R-R interval (heart rate) variation were compared, using a computer technique, in 61 diabetics with a wide range of responses to autonomic function testing. Two methods differentiated best between the diabetics with and without autonomic damage: (1) the standard deviation of the mean R-R interval recorded for 5 min during quiet breathing with the subject either sitting or standing; (2) the difference between the maximum and minimum heart rates recorded over 1 min during deep breathing at six breaths per minute, again with the subject either sitting or standing. For routine clinical usage we conclude that recording the heart rate for 1 min on an ECG, while the subject sits and breaths deeply at six breaths per minute, and then measuring the difference between the maximum and minimum heart rate, is the most practical method currently available. For research purposes either this method or the standard deviation method during quiet breathing for 5 min, should be used.