Evaluation and detection of pathological tachycardias in infants sometimes is difficult. Noninvasive recording of the infant's ECG at the time of symptoms forms the mainstay of diagnosis, although invasive electrophysiological testing may be required. Accurate rhythm diagnosis is central to successful treatment. The advent of high capacity ambulatory memory monitoring makes it possible to record ECG signals, as well as apnea data. We present two cases of infant tachycardia studied by memory monitor. These devices yield good quality ECG signals and may provide information regarding a particular rhythm disturbance.