A combined, cross-correlation procedure for computerized arrythmia detection is presented. It enables the classification of a wide range of arrythmia and is based upon the determination of both P and QRS waves. The electrocardiogram (ECG) signal is recorded from external leads, digitized (10 bits of resolution) at 1.28 kHz and processed with a CDC 6600 computer. A normal beat is stored as a reference signal and the onsets of the P wave and the QRS complex are measured for determining the PR, PP, and RR intervals. The combined cross-correlation procedure is carried out between the template and each successive ECG waveform over two time intervals, the first of which includes the P wave and the second, the QRS complex. Prior to this correlation procedure each of the ECG waveforms is filtered through a nonrecursive digital bandpass filter (10 and 100 Hz for low and high cutoff frequencies, respectively). The cross-correlation functions are then calculated by means of a cross spectrum and Fast Fourier Transform algorithm. The maximum value of the normalized cross-correlation function and the time shift providing that value are searched for, allowing (1) determination of the similarity between the present beat and the normal reference beat, allowing for the discrimination of abnormal P and QRS shapes; (2) accurate measurement of the RR, PP, and PR intervals of this subsequent waveform. Applying this method enables a reliable detection of a wide variety of arrythmia.