Females of both species of chimpanzee, Pan troglodytes and P. paniscus, exhibit conspicuous anogenital sex swellings that are useful external markers for assessing ovarian cyclicity. A new, quantitative method for describing these swelling cycles was refined during laboratory studies of four P. paniscus and nine P. troglodytes and was used to analyze variability in swelling patterns. Clear-cut anatomical changes of the perineum were found to have both interindividual and interspecific reliability. The swollen phase of the cycle was defined as the period when the medial surfaces of the labia minora are pressed against each other so the aperture leading to the vagina appears as a slit (labial occlusion). When defined with this precision, comparisons within and between the species revealed two types of cycle for both species; swollen phases of P. paniscus were about 24 or 39 days in duration, on average, whereas those of P. troglodytes were about 14 or 18 days. Urine samples from the P. troglodytes were collected and analyzed for luteinizing hormone (LH) (34 cycles), estrone glucuronide (E1G), and pregnanediol-3-glucuronide (PdG) (18 cycles). When the two types of cycle were separated in analysis, it was found that the day when labial occlusion occurred served as a predictor of the preovulatory LH peak in the cycles with a long swollen phase. Levels of E1G correlated closely with swelling scores in multiparous females regardless of phase lengths, suggesting that the new swelling scores are a reliable indicator of E1G excretion in multipara. Cycles with long swollen phases had relatively high levels of PdG during the luteal phase, whereas cycles with short swollen phases had relatively low levels of PdG and a shorter luteal phase. The marker of labial occlusion and the duration of the preswollen phase defined by the marker appear to be potentially useful in studies of reproduction in Pan, for monitoring female fertility, and in artificial insemination procedures. If it is confirmed that swelling pattern predicts progesterone levels during the luteal phase, it would make interventions possible in the case of low levels so that a potential pregnancy could be maintained to term.