Ovarian cycles were determined for two captive females of the yellow-cheeked crested gibbon (Nomascus gabriellae) using urinary sex steroids. The mean cycle length was 21.1±1.2 days (n = 7 cycles). The interval between any peak in oestrone concentration and the corresponding oestradiol peak had a range of 0-1 days, and cycle lengths determined with oestrone differed from those determined with oestradiol by 0-2 days. Neither hormone tended to peak earlier than the other. In female 1, menarche probably occurred just before or around the beginning of the colour transition from the black juvenile to the adult yellow fur coloration, whereas the older female 2 apparently began to exhibit regular cycles during this study, years after changing to adult fur colouration. Mean cycle lengths determined in this study for N. gabriellae were virtually identical to those for those of other gibbons determined in previous studies applying endocrinological methods (Hylobates spp.: 20.0-25.4 days, N. leucogenys: 21.9, Symphalangus syndactylus: 21.8). These values are, in most cases, similar to intervals determined between peaks of sexual swellings. On the other hand, published cycle lengths based on intervals between menstrual bleedings or between copulations tend to be considerably longer. Because some cycles may easily remain undetected with the latter two methods, the resulting intervals may not be reliable indicators of the duration of menstrual cycles in gibbons. Cycles of gibbons appear to be shorter than those of other primates, apart from some – but not all – New World monkeys.