Folliculogenesis includes recurrent processes of follicular recruitment, selection, growth and maturation during the female's cycle, ending with ovulation. These process are regulated by interactions between hormones, growth factors, cellular communication systems and genes (Roche and Boland, 1991).Before real-time ultrasonography, knowledge of follicular development was based on the study of the cyclical pa�erns of gonadotropins and steroid hormones, and/or using laparoscopy for counting and measuring follicles a�er marking (Rajakoski, 1960;Dufour et al., 1972;Ma�on et al., 1981;Ireland and Roche, 1983;Fortune, 1993).The ultrasound-scan has made it possible to establish follicular growth pa�erns in two-weekold pre-puberal cows (Evans et al., 1994), in heifers (Savio et al., 1988;Ginther et al., 1989b;Sunderland et al., 1994); in postpartum cows (Savio et al., 1990); in pregnant cows (Pierson and Ginther, 1986;Ginther et al., 1989c;Savio et al., 1990;Thatcher et al., 1991;Ginther et al., 1996), in anovulatory cows (McDougall et al., 1995); and a�er oestrus synchronisation treatments (Sirois and Fortune, 1990;Stock and Fortune, 1993). Rajakoski (1960) described a model of "follicular waves" during the bovine oestrus cycle, reporting differences in the development of a follicular population between one phase of growth, followed by a static phase, and subsequent follicular atresia. Other authors have argued that follicular growth is constant and independent of the cycle (Donaldson and Hansel, 1968;Dufour et al., 1972;Spicer and Echternkamp, 1986).It is now well known that the most frequent patterns of follicular development in cows are the 2 follicular wave pa�ern (Rajakoski, 1960;Mariana and Nguyen, 1973;Pierson and Ginther, 1988;Ginther et al., 1989b;Knopf et al., 1989;Stock and Fortune, 1993; Carriere et al., 1995) and the 3 follicular wave pa�ern (Ireland and Roche, 1983;Stock and Fortune, 1993; Carriere et al., 1995;Taya et al., 1996). Cycles with one or four follicular waves have been reported less frequently Savio et al.,1988; Carriere et al., 1994).Until now, no studies have focused on describing follicular dynamics in repeat breeding cows throughout their natural cycle (without hormonal treatment). Repeat breeder cows (RBC) show a repetition of cycles of normal length and gestation in them that has failed at least 3 times consecutively. The aetiology of this pathology is broad and is o�en described as multi-factor (Lafi and Kaneene, 1988;Bruyas et al., 1993;Almeida, 1995;Pedroso and Roller, 1996). One possible reason for the lack of research in this field may be the difficulty in obtaining a group ABSTRACT: The aim of this study was to examine follicular development pa�erns in eighteen repeat breeder cows through natural oestrus cycles. Ovarian ultrasonographic examinations over 32 days a�er artificial insemination revealed that two follicular waves were the predominant pa�erns in animals with this syndrome (72.2%). Cycles with one or four waves rarely appeared. The ovulatory follicular diameter (day 0) w...