A detector for atrial fibrillation and/or atrial flutter comprises an atrial input for receiving an atrial signal representing an intraatrial electrogram or a time course of an intraatrial impedance, a ventricular input for receiving a ventricular event signal comprising information on an occurrence of a cyclically reoccurring ventricular event in chronological association to an atrial signal received via atrial input, an averaging unit adapted to average a plurality of sections of said atrial signal, each section to be considered for averaging starts or ends at a predetermined offset before a ventricular event, and to put out an averaged atrial signal, a peak amplitude determination unit adapted to determine peak-to-peak amplitude of said averaged atrial signal, and threshold comparator adapted to compare peak-to-peak amplitude of averaged atrial signal to predetermined reference value and to generate an AF warning signal if peak-to-peak amplitude of averaged atrial signal is less than predetermined threshold value.