There are many different serious cardiac arrhythmias. The present invention uses measurements of RR intervals (interbeat intervals) to detect, in particular but not exclusively, atrial fibrillation of a patient. Atrial fibrilation is a serious ailment in which the heartbeat is generally rapid and irregular. Probability density histograms of .DELTA.RRs (difference between two successive RR intervals) collected during atrial fibrillation of a plurality of subjects are used as a template for the detection of atrial fibrillation. In one implementation, there are 16 standard probability density .DELTA.RRs histograms every 50 ms of mean RR interval of a certain number of beats, where the mean RR interval ranges from 350 ms to 1149 ms. Similarity between the standard probability density histograms and a test density probability histogram of .DELTA.RRs of a patient is estimated by the Kolmogorov-Smirnov test. If the test density histogram is not significantly different from the standard density histogram, atrial fibrillation is detected.