The ST-T segment of the surface ECG reflects cardiac repolarization, and is quite sensitive to a number of pathological conditions, particularly ischaemia. ST-T changes generally affect the entire waveshape, and are inadequately characterized by single features such as depression of the ST segment at one particular point. Metrics which represent overall waveshape should provide more sensitive indicators of ST-T wave abnormalities, particularly when they are subtle, intermittent or periodic. This study discusses a Karhunen-Loève transform (KLT) technique for the analysis of the ST-T waveform. The KL technique was used to analyse the ST-T complexes in the ESC ST-T database. KL coefficients were plotted as a function of time, and were effective in detection of transient ischaemic episodes. Twenty per cent of the records showed bursts of periodic ischaemia suggesting local vascular instability. A comparison between kl and ST depression series has shown the KL technique as more appropriate to the study of ST-T complex variations. Using the kl series, an ischaemia detector has been developed based on a resampled, filtered, and differentiated KL series. This technique demonstrates a sensitivity of 65% and a specificity of 54%. These low values can be due to shifts of the electrical axis which are detected as ischaemic changes, real ischaemic episodes that were not annotated with the protocol used at the European ST-T database, or erroneous detections. An increase in sensitivity can be obtained at the expense of a decrease in the positive predictive value and thus becomes a useful technique for previous scanning of the ECG record and subsequent review by the expert. The technique has also been used to monitor patients during a PTCA process, demonstrating that this technique allows us to monitor PTCA-induced ischaemia. A detailed analysis has shown that in some cases a repetitive oscillatory behaviour appears, lasting for a period of around 20 s, and highly related to the oscillatory behaviour of the HR. In other cases, transient changes in KL series with salves behaviour associated with the injection of contrast are shown on the ST-T waveform. We conclude that the KL-based analysis of the ST-T segment is a robust and sensitive technique, with considerable advantages over single feature measures in characterizing the subtle waveform changes which may be of importance in clinical risk detection.