Concealed quadrigeminy has been observed in a 24-year-old pregnant woman without apparent heart disease during 10-hour Holter monitoring; in a 33-year-old female with prolapse of the mitral valve during 0.5-hour continuous electrocardiographic recording, and in an 80-year-old man with anterior myocardial infarction. In all these continuous electrocardiographic recordings 391, 437, 25 consecutive interectopic intervals were correlated and the number of sinus beats in each interval fitted the formula s=4n - 1. The 2nd case showed a variant of concealed quadrigeminy. The number of sinus beats in 9 interectopic intervals fitted the formula s=4n when the first ectopic beat of the interval was interpolated and did not interfere with the propagation of the next first sinus beat.